An exchange should be aware of your age if you have submitted KYC documents. Generally speaking, it will depend on the laws of your country if you will be allowed to sell/buy cryptos. At least in my country you don't possess the legal capacity to enter into binding contracts before the age of 14. If the contract exceeds your economic capacities you will even need to be 18. Buying/selling cryptos always constutitutes the conclusion of a contract.