Voadam
Legend
Depending on what is out there to get xp on, what they know about what is out there, what they encounter, and what options they actually have to choose.If you count XP, then the players decide when they level up by choosing what they do to get XP.
In games counting xp players still can't really just decide they are going to level up every session.
They can decide not to level up by choosing to do things that do not garner xp (although I have played in games where just showing up itself garnered xp or at least some xp).