I did believe it because he was so athletic in Home Alone.1908 Seriously he had some fast feet!

A lot of guys from that era look like that.
I recently watched Die Hard 2. Bruce Willis was only 34 in it and he was like 31-32 in the first film. And he could easily be 40 by the way he looks.
I think it is part of evolution and our lives growing more comfortable from generation to generation.
Right now, a 40 year old guy can pass for a guy in his early thrities.
And this will probably continue and future generations will continually be able to retain youthful looks for a longer time.
Laura:You left a dead prostitute buried alone in the desert?
Kyle:She's not alone.

I think that too.
People looked older than they were back then.
Today, a lot of people look younger than they are.
Maybe it's a combination of the unflattering clothes and hairstyles, and people smoking more/being outside and doing more manual labour work back then.
I actually think Daniel Stern would've looked his age had he not that Bob Ross hair and the funky beard and moustache.

I thought he was near the same age as harry no way he was early 30's he looked mid 40's everybody looked so mature back then nobody in their early 30's today looks that mature people in their early 30's today look like early 20's late teens if they take care of themselves.

Yheah he graduated high school in the mid 70's .They did a great makeup job to get him looking so scraggly and older than he was then