I don't think anyone knows for sure. Rebucketing isn't that well understood on a granular level. Here's an old thread that mentions some of the date cutoffs that have been inferred: http://ficoforums.myfico.com/t5/Understanding-FICO-Scoring/Rebucketing/td-p/268806/highlight/true/pa...
As mentioned....there's no set rule. I might get rebucketed at 12 years and you might get rebucketed at 15.5. It's a YMMV thing and there's no way to predict. It's all based on report content, mix of credit, baddies reporting, AAoA, oldest TLs (overall, revolving, installment), whether or not you applied for new credit in the past year, etc.
You mentioned that you have very late baddies on the SL? If 90 or worse, you won't be rebucketed until those lates go away.
While your oldest account matters a bit, your average age of account matters much more. I don't think oldest account factors into bucketing at all. AAoA definitely does.
Hitting 720 is a reasonable goal. My scores have gotten up to about 750 on EX, and I think the highest I've seen my TU was 728. I have several 120 day lates and a couple 150 day lates.
I would say no. A rebucket can happen if /when something changes the four reasons that are affecting your score. Ie the level of indebtedness goes up or down, A derogatory item being removed from your credit report that was the most recent derogatory item on your credit report. Etc