As in... you don't want them to reach avatarhood until a different level, or you don't want them to reach avatarhood at all, or you don't want them to reach level 65? The first two are easy, you probably just have to change the definition in mud.h for LEVEL_AVATAR to whatever number you want. If its the 3rd, there are discussion about this, just use the search function in this forum.
Well, if you want to to be 100, presumably you want 15 level of imms, so change this definition in mud.h:
#define MAX_LEVEL 65
to
#define MAX_LEVEL 115
since all the other level are defined by "(MAX_LEVE - *)", then that should be the only number you need to change. This will allow players to get to level 100, I beleive. That SHOULD be all that needs to be done, but I don't know off hand. I can think of a where a couple problems might come from, but try that, and get back to us.
P.S. you will have to change the levels of skills and such manually.