If I was you I'ld do this, take the age you mature at, say around 18,and add 1 exp for every day. Someone (I forgot who) did this for experts and it made a lot of sense. Lets see your 25 that means that you have (365.25)x7 exp or 2556xp which would make you a second level character going on to third.
Now, if you considered yourself to be a faster maturer than most, maybe you would consider yourself mature as early on as 15 or 16 which would mean around 3600xp or 3rd level.
If you believe yourself to be a better learner than everybody, maybe 2 or 3 xp a day would more suit your character which would mean you have around 6,000xp or 4th level. And should you be a child genius, with extraordiary abilities at learning maturing ect, your looking at around 10,000 exp at 25 (using a maxed out normal progression) which means 5th level.
Of course this doesn't account for accelerated xp awards, for instance having a rough childhood, or being stranded at sea, or killing a dragon.