What is the minimum age a person must be to use the Bitcoin service?
There is no minimum age, my children have Bitcoins and have for years.
Service by service should implement whatever safety or age restrictions they need.
I am not aware of any work done on this front, perhaps somebody else can enlighten.
Some merchants, exchanges or E-Wallets even may have age restrictions, but Bitcoin is a protocol and therefore there is no restrictions regarding how it is used.
Unless you are a miner, you do need to buy Bitcoins; and given the currently limited range of Bitcoin-only services, you do need to sell Bitcoins as well.
While neither buying or selling Bitcoins has an explicit age restriction any more than holding and spending pocket money at a car-boot sale does - if you buy or sell Bitcoins from a bank account you will be constrained by the age restrictions that may apply.