Hey Shekhar, what is your regret about? About not getting your citizenship earlier? Or is it just you had some interest in military but didn't have enough courage to join back then and now you think its too late to join? If its just the citizenship, you are on your way to receive it. If you still have some interest in joining military, US military is very lenient on age limit. Since you are permanent resident already, you can go National Guard or Reserve to just to give it a try. Just sign up for minimum possible years and make sure to pick the right specialty when you sign. You could get your naturalization expedited too potentially if you join military.