The Search For Minecraft’s Long-Lost First Account
Any account created after late 2012, or one that has been migrated from the old account format, is a Mojang account. With these accounts, you log in with an email address. Older Minecraft accounts that have not yet been migrated use usernames to log in.
An older Minecraft account that has not yet been migrated uses the username to login. To reset the password for a Minecraft account (you log in with your username) you will need to visit account.mojang.com/migrate and migrate your account to a Mojang account. After doing this you will be able to reset your password.