For men in movies:

Sean Connery was 31 in Dr No. Leonardo DiCaprio is now 47 and STILL looks younger than him.

I think health and beauty standards have definitely played a big part but I think the biggest part is just the nature of casting.

The traditional leading man was a tall, dark, distinguished and classically handsome gentleman.

During the 80s, casting started to shift towards 'fit, healthy, young and angel faced boys'.

Besides character actors, this trend of casting has become more and more prevalent.