A Jason or an Alan/Allen would be about 50. The Alans/Allens might even be older too.
There is no guide to names unless you want to look up names from different decades on google.
However, people here have a variety of different ages and are familiar with the names of their classmates from school and their friends at university and colleagues at work, and they know how old these people are. They also have children of different ages, and are familiar with the names of their children's classmates.
Names come and go according to fashion in the west, so a person's age can sometimes be guessed when you see or hear their name.
I am quite often struck by the incongruous choices many Asian people make when picking a 'western business' name -- young women choosing names like Sandra or Cheryl, young men choosing names like Floyd or Herbert..
If your target audience is the US, I would pick something like
James (Jim or Jimmy for a nickname/short version),
Matthew (Matt for a nickname/short version),
Daniel (Dan or Danny for a nickname/short version),
Christopher (Chris for a nickname/short version),
Michael (Mike for a nickname/short version)
Peter (Pete for a nickname/short version),
Joshua (Josh for a nickname/short version)
Jacob (Jake for a nickname/short version),
Ryan
Joseph (Joe for a nickname/short version),
Justin
Alexander (Alex for a nickname/short version),
Marshall
Eric
Kyle
or Scott.
Most of these would work well in the UK too -- maybe not Scott or Kyle though.