Class based characters are not created equally. That is to say, I can see that characters of differing classes will start at different ages. The mage, who locks himself up in the library, studying spells, is very likely to be older (and consequently frailer, which the mechanics of the system do bear out) than his fighter friend, who may have just ended his military service. With this in mind, I have created the following table:
|Class...||Starting age...||Learned trade in...|
|Fighter||d8+18||Military school, guild or in battle|
|Magic User||d20+30||Library, apprenticeship or in self study|
|Thief||d6+14||The streets, thief's guild, or the marketplace|
|Cleric||d12+25||Temples, holy tomes or in silent contemplation|
Note that I haven't included ages for demihumans on the table, at the moment we have none in the party and I'm a little unsure about their lifespans. I suppose I could consult my 2e PHB which I still have from all those years ago, but for the moment, this should suffice.