That 20-40 range seems about right. It gives a bit of flexibility so you can imagine your PC is at the younger or older end of the scale and it still won't seem odd in terms of conversation, companion reactions, etc. Any younger and I would imagine there would be comments about their age, which would probably also be the case if they were very much older.
I always imagine the Warden to be in late teens, early twenties. In the Mage origin you are just taking your harrowing, which happens pretty early on. The reason Jowan is getting so jittery is that he is getting a bit old for someone who has not yet done it. The City Elf is about to be married and it seems likely that this is encouraged pretty early. Since the Dalish Warden had not yet married, I took that to mean they were still quite young. I'm pretty sure that a marriage would have been arranged for the young Cousland if they were any older since marriage is generally about cementing alliances. In fact I recall that there had been talk of male Cousland with Delilah Howe.
Hawke starts off in their early to mid twenties, which would make them in their early thirties by the end of the game.
I think I shall play my first Inquisitor as late twenties/early thirties so they have a bit of experience of life and it makes it believable if they seem to be able to cope with the role thrust upon them. However, if I find that the dialogue seems to allow for someone a bit younger and inexperienced, who is not so comfortable in the role, then I'll try that second.