Sustaining 15 years at staff level is near-impossible and will extract a heavy toll for the vast majority of people. Plus at 55? Not saying you'll be dinged purely for your age but it's not like it has no influence ...
That’s not at all what I’ve seen. If someone’s properly leveled at Staff, it tends to be a bit easier to sustain than Senior in the same company, but you do have to be selective about new roles at new companies. Age shouldn’t be an issue for someone with Staff+ skills except in the most dysfunctional, unprofitable companies.