When you create a character, if it's older than 20 years old, it receives 1 setback and +1 to one domain for each step of 5 years old, starting at 21.
Means this:
21 <= age <= 25 : 1 setback, +1 to one domain
26 <= age <= 30 : 2 setbacks, +1 to 2 domains
31 <= age <= 35 : 3 setbacks, +1 to 3 domains
You can extend this table for older ages, if you want a basic rule:
36 <= age <= 40 : 4 setbacks, +1 to 4 domains
41 <= age <= 45 : 5 setbacks, +1 to 5 domains
46 <= age <= 50 : 6 setbacks, +1 to 6 domains
But this is not really well balanced for a PC, as this is for creation, so you may need more XP, because of the adventures he lived, the background he has, and +1 to 4 or 5 domains is not clearly balanced with what a normal PC would gain as XP.
We can then change the table, decreasing the "age step" to 10 years instead of 5, but adding 10 XP for each step (remember that +1 to one domain costs 10XP, so increasing the gain is more relevant for an old character), and keeping the previous gains.
There is also a problem you cannot solve during a game, but you can when creating a character and with this system : advantages and disadvantages. Normally, you would not gain neither adv. or disadv. during a game except in very special conditions, but as we're creating a character, we will allow the character to have more advantages and disadvantages than a "normal" character, which helps finding where to spend the XP gained. We can rise the max number of adv/disadv to 5 instead of 4, and also we can rise the max number of "double-selected" advantages ("Strong"<"Very strong", etc.) to 2 instead of 1 (not including disadvantage "Trauma", of course).
Then there is the theorically final table:
21 <= age <= 25 : 1 setback, +1 to one domain, +10 XP
26 <= age <= 30 : 2 setbacks, +1 to 2 domains, +20 XP
31 <= age <= 35 : 3 setbacks, +1 to 3 domains, +30 XP
36 <= age <= 40 : 4 setbacks, +1 to 3 domains, +40 XP
41 <= age <= 50 : 5 setbacks, +1 to 3 domains, +50 XP
51 <= age <= 60 : 6 setbacks, +1 to 3 domains, +60 XP
(the values are expressed as "total bonuses/maluses", not "bonus added since previous threshold")
As there are many setbacks for an old character, I think you may not accept dices roll of 1 and 10 more than once, especially as there are 8 "real" setbacks (score of 1 adds another setback, and score of 10 makes you "escape" a random setback). If the character is both lucky and unlucky, (or if the GL is particularly evil

) the character can have all 8 setbacks, and rolls/chooses the one he has escaped by luck ! This would really make a big background for the character
Hope this "test" solution helps ya creating old characters !