I think I wrote about it above and the way you replied it feels like you didn't just run into it today. I've been bothering you.
I'm going by my eyes. Kirk is a young Captain in TOS. Scott is a seasoned hand, well into middle age. He's just one of those solid backbone officers that a young Captain needs to rely on. To me this is deep into his character and role in the crew. Especially as you proceed into the movies, it never isn't visually obvious that he's an older man and not just a little bit.
And the issue isn't just his age. There is not actually much time for this baby-faced gee whiz-kid junior lieutenant to transform into the steady, firm-gripped Lt Cmdr Chief Engineer of the Enterprise. And when did he serve on the freighters and stuff? This actor is 31 and looks younger if anything.