Assuming you were 22 on day 1(like 3 of my own players), by boosting to 25 on day 1, you would cost yourself 400 exp (less than half a level, roughly 2 SP), while gaining 4 vet points in this season. 4 vets that you can never get, as opposed to half a level (which, depending on exp, you may not get anyway ever). Starting at 23, you'd lose 300 exp, get 2.75 VP, staring at 24 you lose 135 for 1.5.
Math gets complicated for mid season, but essentially, you're better off boosting to 25 as soon as possible.