A good candidate for hair transplant surgery would be one who is suffering hair loss due to male patching baldness. Some one who has lost hair and has just a ribbon like or band like hair at the back of his head.
What can be done is to have dense patch up done so it looks natural and not bald. It’s not possible for the candidate to expect the looks of a teenager with full headed hair.
The results of transplantation will vary however from candidate to candidate. The most important fact is to consider if the candidate is a good one for transplantation who can achieve reasonable results as he expects.