I think the average matriculant is ~24-25, though this varies from school to school. I heard from several people throughout the interview process that being slightly older and/or married is viewed quite favorably. At the dental school level, professors do NOT want to deal with the issues of people who still think they are adolescents. 🙂
I hope age is not a big deal. By the time I enroll, I'll be 28, married, and have 3 kids. I hope Adcoms don't think I've got too much to handle already.
The ADEA Dental School Guide has the ranges of ages of accepted students; the oldest I noticed was 46 y.o. I nearly dropped dead. That was at a state school (Illinois), but it sure made me feel better. A few schools have a range like that. I think older than early 30s is not very common since people have other priorities by then. Namely, a family.
I think age discrimination is illegal by federal law, so it shouldnt be a problem. They will definitely bring it up if they grant you an interview though.