Sam's age was frozen for Double Agent because otherwise they thought he'd be way too old, but they kept his birth date at 1957 when Conviction was released, so there was no real change before now.
Conviction was also bad, I played it but compared to the other Splinter Cell games its no where near as good. I've even seen it labeled as just 'Conviction' a few times without the Splinter Cell prefix, which feels right to me because it isn't Splinter Cell.
Go onto the Ubi forums and all you'll see is others sharing the opinion that SvM should've been in Conviction, as well as the fact that the game needed to bring back the stealth based gameplay from before. That and having the HD collection go multi-platform but thats about it.
Achievements aren't a factor in this, I have played a lot of games to completion and not all of them were amazing to me, I have a lot of criticisms about most games, Blacklist just stands out more on the list. Conviction was just an average game, I played it and completed it and then just wished that Ubisoft would make a really GotY for once.