There aren't that many bike rental services around here. I've looked in the past and I was 24 years old at the time, so age was not the factor that prevented me from being eligible. It was more the lack of options and the high price of rental that made me lose interest.
It was expected though. If there's only one place that rents bikes, they don't have to compete against any other company so they can charge higher prices.
If someone approaches you and says they'll rent out their personal bike to you, it could be very dangerous for you should you be involved in an accident. Most bike insurance policies have them setup as "personal vehicle for pleasure only". If someone rents out the bike, it becomes a commercial use and is not covered under the insurance. If you hurt yourself, someone else or wreck the bike, it's all coming out of your pocket.
Insurance specialists are definitely welcomed to correct me if I am wrong.