The way we have been handling aging is as follows (note the number ranges are inclusive, counting both first and last year listed.)
20-25, 6 mods over 6 years. One increase per year rolled randomly w/ D6, re-roll if already increased during that bracket.
26-31 5 mods over 6 years. One increase per year, randomly w/ D6, rolling a 1 is a ‘bye’ as above re-roll if already awarded.
32-41 3 mods over 10 years. Roll a D10: 1: -50 dex, 2: -25 cha, 3-9: ‘bye’, 10: +25 will. re-roll if already awarded (except bye), once 7 byes handed out, it becomes a re-roll.
Obviously if your number of choices gets small, you can use smaller dice to avoid a lot of re-rolls,
Above that we have not dealt with, but the same basic format can be applied.
And yeah, you need to keep track of these somewhere (back of char sheet, etc) as during each phase a given adjustment is awarded only once.
We have allowed ‘banking’ of XP at character creation in the form of CP allocated to attributes to allow for taking advantage these adjustments. If you are creating a character who will be over the age of 19, you can allocate CP to your stats in anticipation of aging adjustments, eg 750 towards Ref would give you Reflex 7, but it would jump to 8 when you got your aging adjustment for that stat. However the aging adjustments are rolled after character creation, and after your bonus roll, so have a good crystal ball at hand.