Q: can u start developing man boobs at the age of 12

A: Yes, you can. It can happen at any age if you have a poor diet and/or are overweight. It's especially common for teenagers, even those who are not overweight (though it usually tends to happen a little later than 12 years of age). In either case your continuing growth and development will take care of them with time, but you can definitely help along by eating healthy (lots of vegetables and whole fruits) and being physically active.