At why age can you donate a kidney?

My grandpa went to the doctors Friday and turns out he has stage four kidney damage and has 6 months to 2 years to leave on dialysis. Well turns out on my grandpas side of the family know one can donate because they have diabetes ms and schizophrenia. I want to donate my kidney to him plus I'm a match but I'm 13 and I don't think age matters or does it?