Your birthday will not be visible to others on Instagram, but you’ll be able to see it when viewing your own private account information.
If you’ve connected your Facebook account to your Instagram account, they will add the date of birth that’s on your Facebook profile. This is not public information and only you will be able to see it when viewing your own account information on Instagram. Editing your date of birth on Facebook will also change it on Instagram. If you don’t have a Facebook account or if you have not connected your accounts, you can add or edit your birthday directly on Instagram.