I'm 24 years old. At what age i should do hair transplant? (photo)

I suggest seeing a hair loss expert now to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

According to your picture, you need about 2,000 grafts to fill the temporal area. The procedure takes about 6 hours. However, you're still young and there's chance to you will lose more hair. So, you're recommended to see qualified hair transplant surgeons for medicine treatments alongside the surgery to prevent further hair loss. 

Its difficult to make an estimation from a picture however you do appear to be thinning over most of the scalp. Seeing this, I would recommend visiting a qualified hair restoration physician as soon as possible to analyse your hair up close and review your family history. No one has a crystal ball to see into the future but there are certain clues in one's history that can help your doctor estimate how rapid and extensive the balding process could be. Answering this question first will allow your doctor to be able to answer your other question, at what age should I get a transplant. 

You should first get a HAIRCHECK test to find out how much hair you are losing that you can’t see as that will point to the risks of further hair loss acceleration with a hair transplant