The only reference that I can find regarding the age of the SLN's Scientist and Vega classes is from:
Mission of Honor, Chapter 20 wrote:There was an impressive uniformity among the superdreadnoughts, as well. All but seven of them were Scientist-class ships, and all seven of the others were members of the Vega class, which were basically only repeat Scientists with a couple of additional missile tubes in each broadside. By the standards of the prewar Royal Manticoran Navy, they weren’t that bad a design, although the first of the Scientists had been built long enough ago that they’d still been equipped with projectile-firing point defense systems. At least all of these ships seemed to have been upgraded to laser clusters since, judging from the detailed passive scans Augustus Khumalo’s Ghost Rider platforms had pulled in. And it was painfully obvious that even now the Sollies didn’t begin to grasp just how capable—and stealthy—the Ghost Rider recon drones actually were. To be sure, the really close passes had been purely ballistic, with no active emissions to betray their presence, but even so they shouldn’t have been able to get in close enough to literally read ships’ names off their hulls without someone noticing something.
Italics are the author's, boldface is my emphasis.
Point defense using autocannon started to be replaced by PDLCs before the time the laser head missile came into use--the introduction of the laser head accelerated the process. Some information, including relevant systems and dates:
The Short Victorious War, Honor Harrington's Navy, Naval Weaponry wrote:In some navies, the lasers were backed by a last-ditch autocannon defense. The theory was simple: throw so many shells that they built a wall of metal in the missiles' paths. Given missiles' closing velocities, any hit could be counted on to vaporize them, but the development of laser heads made autocannon largely irrelevant. When a missile can attack from 20,000 or 30,000 kilometers, no last-ditch ballistic projectile can reach it in time.
In Fire Forged, An Introduction to Modern Starship Armor Design, Deep Space Warfare from 1246 to the Present: A Short History of the Threat wrote:The gods that govern arms races abhor imbalance, and the sidewall’s impenetrability did not last long. In 1298, research yielded the first practical sidewall penetrator.
***Snip***
Spacecraft designers responded with vigorous innovation in defensive measures. Early research in electromagnetic and gravitic deception and countermeasures renewed the importance of the ancient and archaically named art of “electronic” countermeasures. Navies turned to mass driver technology for point defense. The point defense autocannon found in some third rate navies today, and even on truly ancient reserve Battle Fleet vessels in the Solarian Navy, are direct descendents of weapons developed in this period as a response to the early contact nuke.
***Snip***
The defense was not at all idle during this period. Advances in gravitic deception technologies raced neck-and-neck with seeker improvements. Sidewall systems largely took the lead in thwarting penetrator improvements and improved materials and designs kept the defense almost in step as missile warheads grew. The pure fusion warhead might have had more disruptive consequences if the impeller drive countermissile had not appeared on scene in 1701. Essentially a smaller version of the shipkiller, this weapon destroyed incoming missiles by wedge to wedge interaction. This added a new depth to the missile defense problem which allowed nearby ships to defend each other cooperatively as never before. The countermissile dramatically reduced the effectiveness of shipkillers. This was followed some eighty years later by the widespread introduction of numerous small point defense laser weapons. The new active defenses ensured that even a weapon whose seeker was not decoyed by the target’s ECM would be stopped short of the sidewall. The point defense laser cluster created the final layer of light speed defense that resulted in the now familiar geometrically increased chance of the missile being destroyed in the last 50,000 to 60,000 km of its run. Hits against intact defenses became rare. This relegated the impeller drive missile to a counter sidewall role in which the best that could typically be achieved was a close aboard detonation of a multiple missile salvo to burn out sidewall generators and soften the target up for an energy range attack. Sidewall burning was in fact the end to which the largest pure fusion weapons were built.
***Snip***
By 1826, a state of the art RMN impeller drive nuclear armed missile could boast a standoff range of 8,000 to 10,000 kilometers in sidewall burning mode.
That same year, a small Solarian defense contractor, Aberu and Harmon, developed the unique combination of a state of the art grav lens array with a series of multiple submunitions carrying rods that emitted short wavelength X-ray laser light when exposed to the broadband X-ray pulse from a nuclear explosion.
***Snip***
Initially called a laser enhanced nuclear gravitically directed energy weapon (LENGDEW), the device quickly earned the handier title of “laser head.”
***Snip***
Strong Aberu family influence within the SLN and the lure of being first eventually won modest funding for a short series of live tests in 1833.
***Snip***
The tests proved embarrassing failures.
***Snip***
The resulting media attention brought word of the laser head briefly into the public eye where it came to the attention of Astral Energetics, Ltd. Sensing an opportunity, the company bought out Aberu and Harmon, collected all existing research materials, and began a long term incremental development program. Astral’s huge sales of gravitic and nuclear physics packages for military, mining, and scientific uses meant a steady flow of resources to their extensive project team located amidst the sprawling industry of the 70 Virginis system. Their work took over thirty years, but it produced the first complete laser-head-armed impeller-drive anti-ship missile system in 1866.
***Snip***
Convinced that the weapon was nothing more than a passing novelty, the SLN rejected the best efforts of Astral’s sales department and lobbyists for years. Desperate, Astral eventually began advertising the weapon for export. The Imperial Andermani Navy was their first official buyer in early 1872. Successful, though infrequent, use of laser heads against pirates in Silesia over the ensuing decade encouraged the People’s Republic to begin acquiring laser heads and the capability to produce them in the early 1880s in the midst of its forcible expansion into much of the Haven sector.
The Star Kingdom of Manticore pursued an independent path to laser head armament. Always admirably well informed on galaxy-wide research trends due to command of the Manticore Wormhole Junction, the Bureau of Weapons (BuWeaps) presumably learned of the laser head concept when it first became public knowledge in the late 1830s. Thus began a low-level development effort which confirmed the validity of the basic physics without developing a functional weapon. Even Manticore’s vaunted research and development establishment struggled with the complex problems of gravitic technology miniaturization, timing, and nuclear processes for many years. Manticoran work paid off in 1870 with the introduction of their first laser head capable missile—the Mark-19 capital ship missile.
The advent of the laser-head armed impeller drive missile put a premium on keeping enemy missiles far away from one’s ships and forced defensive system designers to make dramatic improvements in countermissiles, point defense laser clusters, gravitic sidewall strength, and armor.
Boldface and underlined text is my emphasis.
Conclusions about the age range of the Scientists and Vegas:
The earliest SLN Scientist-class SDs predated the use of PDLCs and used autocannon point defense, while the later ones built (plus any later refitted and modernized) used PDLCs instead of autocannon.
Since "the widespread introduction of numerous small point defense laser weapons" occurred (approximately) in 1781, the earlier builds of Scientists with autocannon point defense occurred before ~ 1781, with the later builds of Scientists starting on or after ~ 1781 used point defense laser clusters instead.
SLN Ships undergoing refits and upgrades after 1781, and especially after 1872 when the laser head was successfully used, would have their autocannon point defense replaced by PDLCs.
The Vega was a repeat of the Scientist, with some additional missile tubes. Most likely, the design entered production after the last of the Scientists ordered were starting construction.
