Obligatory "not a lawyer." Just my guess.
If Roshi can hide the fact that he's immortal then he could get a lighter sentence. If the court knows he's immortal then he should likely get no leniency.
Considering Toriyama must have been thinking of Japan's laws at the time, Roshi being interested in 16-year olds would not be entirely illegal under those laws. A faux-pas, but not barred by law.
Asking to see the undies of a 16-year old, I don't know. Bulma didn't know she was going commando so the fact Roshi saw more than he thought he would probably wouldn't be counted against him.
In other cases, has has committed non-consensual touching, so and in total probably several years.
If you apply modern laws he'd probably go away for decades.