The age difference matters not. I pledged an undergrad fraternity when I was 21 and rushed in the Fall. My grades might have suffered a little bit, but I was a little irresponsible to begin with back then and quite frankly a heavy partier also. Actually my grades suffered more as a brother, but I did eventually get my act together and there were people who pledged and did very well academically (Deans List and all the rest), so it all depends on you.
Most fraternities are very dedicated to their pledges doing well academically. As far as time commitment goes, it depends on the fraternity. You can expect there will be a 1-2 nights during the week you may have to devote some late night hours (11pm-1am) to pledging. Weekends you will be busy pledging (setting up/cleaning up for house parties/mixers along with other tasks). Occasionally there may be a mixer during the week. It really depends on the Fraternity.
Don't pledge for access to women. Sure that will certainly happen, but the biggest benefit is the close bond of brotherhood you will form, and that lasts a lifetime.
Well worth it in my opinion for a slightly lower semester GPA, and that's only if you aren't a good academic already and it sounds like you are.
Pick a fraternity where you like the guys, but be mindful of when they are graduating. The ones you will be hanging out with the most are not the ones who will be graduating next year. They will be the younger ones and most importantly the other guys rushing. They will become some of your closest friends whether you like it or not.
Ask brothers about the pledge program. Truthfully, you want a hard program where there is a good amount of hazing because people bond closely through common suffering. You will be bound to your pledge brothers through it, and to those before you who went through it and those who come after you. Especially the ones you haze personally. It's a special kind of bond, that's hard to describe.
If you end up becoming a brother you will learn there are guys who are pro-hazing and those who are anti-hazing and there will always be a kind of tug-of-war within the fraternity over it. I am pro-hazing.
Personally, I may not join a Fraternity where excessive alcohol drinking is part of the "hazing", or where they hit their pledges. I don't see anything masculine about striking a defenseless person who can't hit back, or forcing someone to drown their awareness and risk their health.
We never did that and believe me there are soooo many better ways to motivate pledges.
Fraternities should be quick to tell you if they don't force drinking and they don't hit. Those are selling points, or at least they should be. Those that don't, you can probably assume they do.
Finally, when your rushing, never refer to a fraternity as a frat. You don't call your country a cunt, right? That's the first thing you'll be taught, and the brothers will notice when you say that word.
Good luck with it, and if you pledge, embrace it and have fun with it. It's something you will never be able to do again.