If you want to fight amateur you might need to check the rules RE age limits with the boxing commission in your area. Where I am the max age is 33 to compete in amateur boxing.

Apart from that I'd find a boxing gym, start training and when you're feeling fit and strong and discuss it with your trainer. After all if you get to that stage you've still achieved a lot, learnt a lot, become much healthier and gotten to see what a most of it is about - the training! You'll also have a better idea of how much time you'd want to put into your training and whether training to compete is going to mesh with the other priorities in your life.