As far as your age is concerned, lets look at this pragmatically.
You compare yourself to a 20 year old. While at your age, thats a reasonable difference, but look at the big picture. If you stayed in for 20 years, you will be 45. Compare that to the 20 year old... who will get out at 40. The difference between 40 and 45 is significantly different than 20-25. Who knows, if you went officer or made rank faster, you could possibly be even better off than the 20 year old.
At 25, you SHOULD have a maturity that will help stand you above your fellow sailors that are younger than you. While this may not give you the "oh wow" factor you desire, it can help you succeed where the younger ones/"less experienced" may not. *NOTE* please understand my "should" and "can" are not to be implied as "are" and "will". Im just relating myself now to what I was when I was 18-24 in the navy.
The real detriment is if you DON'T stay in the navy. If you do six years and leave, you will be 31, not 24-26 like most of your constituents. this can make it a bit more difficult to get on track with other goals you have for yourself.
Basically to sum up all this, you really need to do some soul searching of what you want to do with yourself. Being a "nuke" sucks while you're in. Theres really no bones about that. If you think 60 hours are demanding, how about 120? No, thats not an exaggeration. Some weeks were 140.
Finally, with power school, theres absolutely nothing to learn. In fact, you're probably behind if you have taken significant math courses. You LITERALLY start from 2+2=4 and build from there. They will teach you everything. For now you have to focus on passing your NAPT, and do not study beyond that test. Nothing you study now will help you for power school.