Gen I: After awakening Snorlax in the way, that means at least Surge defeated (third badge) AND the Marowak ghost events cleared.
Gen II: After defeating Chuck (fifth badge)
Gen III: After defeating Winona (sixth badge)
Gen IV: After defeating Maylene (third badge in DP, fourth in Pt)
Gen V: While no longer requiring any badge, it's obtained around Driftveil, that means unobtainable before defeating Elesa (fourth gym, in both BW1 and BW2)
In all of those instances, your team should be already around level 25-30, and every "early-bird" Pokémon evolves around level 15-20.