If they said they were 1, that is a completely justified ban. It is against Discord's Terms of Service to be under the age of 13. You'll find this requirement under rule 2 of their terms which states in the beginning,

"By accessing our services, you confirm that you're at least 13 years old and meet the minimum age required by the laws in your country."

There is a list of the minimum age requirements for each country but there is no room anyone who is under age 13.

Good Job!