I posted this in another thread, but I feel like it's interesting enough for its own topic and less of a tweak than a change.
One of the main things making Mabinogi so difficult(Especially for free players) is the amount of time you need to spend to get anywhere in the game. You shouldn't need to wait 3 weeks in between each rank of a skill like Ice Spear, or Crash Shot, etc.
Basically, the revision I proposed would do the following: First, the free rebirthing at age 20 would be removed. However, it would be replaced by the ability to free rebirth when you've reached or passed a designated current level, starting one week from your last rebirth.
The designated current level would be decided from your character's age and its cumulative level at the last rebirth. For example, a brand new total level 1 player might have to reach current level 5 or current level 10 to earn his/her free rebirth, while a total level 3000 character might have to reach current level 80 or 100 or so. I figure that the current level required would begin to lower starting at age 18 and would be pretty much the same for all the ages before that.
For every level you get past that designated level, you might be able to free rebirth a little sooner than a week after your last rebirth(Though this might be set at a cap to prevent abuse of sorts, like a maximum of 2/3 rebirths in a single week, and wouldn't apply to paid rebirths).