Please don't question my curiosity.
Gen 1 - level 30 Gen 2 - nah. Gen 3 - level 20 FR/LG - lvl 5 Gen 4 - level 20 Gen 5 - level 25 Gen 6 - level 20 Gen 7 - level 15
Source - some from Serebii.net some from bulbapedia and some from experience.