I think you will find that there is actually a fairly wide range of ages at the Lt. and Cpt. level in the reserves - it really varies from unit to unit.
As a rule, as Horse Solider noted, it's true that most are in their 20s - but there is also a significant number who are older. (I happen to be one of them)
There is also what I call a militia "sub-species" of 2Lts who are in their 40s-50s who often do very specilized roles (Fin O, UIO, LogO, Admin, etc - or some form of duty thereof) , but because of work pressures can't get enough time off to get fully MOC qualified quickly - or are taking years to get MOC qualified because they are using the two week block system.
They are often dedicated paraders who do paper work or other niggly tasks that the younger guys might not be especially keen on (who can blame them) , but again, getting anything beyond two weeks off at a time for training is problematic for them to advance in promotion.
Some 2Lts. have decided to come back to the reserve after a long hiatus and decided to re-enroll; and some are CFRs. (Interesting to note that in my unit I would say we have a generation gap with hardly any captains who are in their late 30s-40s - probably because these are prime years for career development and family)
If you're worried about being too old for the rank - don't be. As long as you pull your weight, your fellow subbies will respect you no matter what your age.
As for comparisons to other militaries I don't know. I think there are probably a lot of older guys in the USNG from what I've seen on the news but that's just anecdotal.
cheers, mdh