Interestingly, the voting age was reduced from 21 to 18 around the time the story was published
We were having a discussion about Bruce and Dick's relative ages on another thread where someone mentioned that societal perceptions and expectations of who is a ''fully capable and independent adult'' have shifted over time. Whereas a 20 year old today would be regarded as a kid, a generation or two ago, 20 year olds were fully-grown adults with all the behavior and responsibilities that carries.
In this context, Bruce being 20 or 21 when he starts out as Batman made perfect sense in 1939 or even in 1969. Today, the age of 25 seems a lot more appropriate. And the movies tend towards an even later age - in the Nolanverse Bruce started out as Batman
just before his 30th birthday, I think I've heard it mentioned somewhere that Pattison's Bruce in the upcoming film is around 30 in his second year as Batman, and I'm pretty Michael Keaton wasn't playing a 25 year old in the first Burton movie which was also pretty early in Batman's career.