What happen if a boy starts masturbation at the age of 13 on a regular basis and now that boy is of 18?

Nothing specific. Masturbation is a normal part of human exiatance.Some cultures frown on the process & most seldom discuss it openly. If anyone does so to the exclusion of interacting with people & pursuing age & peer related activities, it can interfere with normal emotional growth, as any preoccupation might. For most, it is simply part of their life.
Normal. If a boy does that then he will be a normal boy just like every other boy. Nothing will happen other than he be normal.