An in-campaign option would be for elemental races to gain advantages while they develop. So, a starting player would (to use munchy's example) only have a special racial bonus, and maybe a reduced affinity talent (only +10 on resistances, no casting bonus, no DB) for, say, 10 DP cost.
Then, by time, experience or ritual, individuals gain maturity later on, pay the other 20 DP and have the full affinity.
I do like the "reduced DB and resistance vs opposing element" option. That would also be a thing for starting players to suffer, but to overcome by coming of age.
Hope that helps - to me, it seems very fitting to have elemental powers grow and evolve, as they do in nature. Also, you could surprise your players with new elemental options later on (a "free" spell (treat as +1 spell adder) with a fitting spell, e.g.).
On a sidenote: You could go with a "every 5th/ 7th level" thing, and either swap that "ability" for any professional ability a character might have, or make "earth elemental gnome" a profession.
Cheers,
Jan