Never wear a shirt from a run that you did not finish. To wear it is to say I finished it. During a race the wearing of shirt from a previously completed year is acceptable. This is probably a good practice because you now have no excuse to drop out since you've done it before. T-shirts which can be worn without regard to running the race.
Volunteers have full T-shirt rights and all privileges pertaining thereto.
No souvenir shirts therefore friends or anyone else not associated with the race may not wear a race shirt. If mom thinks that the Leadville shirt is lovely, tell he to send in her application early for next year so she can earn her own. Wear the race shirt of your last race at the current race pre race briefing. The more recent the race the better. This is a good conversation starter.