Skywest won't upgrade or transition you between planes if you've got less than 18 months before mandatory retirement. More than 1 Brasilia pilot had to retire early when that plane went away.
Don't know about anything in the PPM about new hires and age, and it could be an age discrimination lawsuit if someone wasn't hired at 63 yrs 7 months, but I'm not sure commuting a year as a jr reserve at 64 would be worth it.