I believe the discounted red tickets are also valid for youth 12 and younger (if you're traveling with kids)
^ I just looked that up on BART's website.
BART Red* Ticket
A 62.5% discount for persons with disabilities, Medicare cardholders and children 5-12 years old (starting January 1, 2018 the youth discount will extend to 18 years of age and will be 50%) : $24 ticket for $9
https://www.bart.gov/tickets