At 38 by RE1(1998), I think he's really developed all he's really needed to. 48 by RE5(2008 or was it 2009 I think they changed it).
Anyway, was he going through a midlife crisis? I mean, it's not like he was a teenager becoming a man, or a prebubescent becoming a teen...it was an older man. Senile was a word used for him. That's not a word used for young people.
Anyway, the development was odd, annoying, didn't make any sense, and I hated it. Claire's development was great, Leon, up to RE4 anyway, Chris, Jill, etc. Wesker though, not at all.