What advice would you give someone who is joining the National Guard at 29 years of age?

I'm going into the Navy at almost 27. I think there will be a lot more pressure for us to be more mature and responsible especially at boot camp. I know I plan on stepping up and I'm going to try to be a leader at boot camp. I don't really have any advice because I'm in the same boat as you but we just need to act our age and not join in the high school drama of the younger recruits. lol

I was 27 when I went through OSUT, 29 for OCS and 30 for my Officer Basic Course. Stricter on entry than when I entered, so get in shape and make sure you can make weight when you got to the MEP's. If you are in shape, Basic isn't terrible and they tend to pull the older guys for leadership positions. Listen to the drills, do what they say and it will pass quickly. AIT for a 25B shouldn't be a big deal with your background and it should be considerably less physical than being a Grunt (Infantry, which I volunteered for twice).

Utilize this website for the physical fitness standards:
https://usarmybasic.com/army-physical-fitness/apft-standards

Locate your age bracket and the minimum score for AIT is a 50 in each event. Once you get to the operational NG, the minimum score will become 60, so my recommendation would be to train for achieving 60 percent or higher in each category. As for any other prep, there is really nothing to advise on that front. AIT is designed to take the slowest learner and make them proficient in whatever the MOS is. You have a severe leg up on the competition due to your background in Computer Science and InfoSec so don't sweat it. Get into the mentality that you don't know everything and you can learn something from anyone. Its not a good idea to show up to AIT with arrogance. Absorb what you can and strive to learn new things every day and you should be fine.