Is there a forced retirement age for Part 135 pilots?
There is no FAA mandated retirement age for part 135 operations, although there was in 2017, a proposal in Congress that would limit the maximum age but only for certain operators. As of the date of this answer there is no regulation for 135 pilot ages.

It's up to the operator then to decide if they want to hire somebody who can only fly domestic routes. For some this may not be an issue, for others it may be a deal breaker.