"You can join Teen Second Life on the same page as the Second Life for adults. On which of these variants you'll get after subscribing depends on the real life date of birth you're giving there. (Note that the Terms of Service require you to give true and accurate data when subscribing, so you can't choose between teen and adult Second Life.) For neither you are required to give a mobile phone number.
Age Verification is one of the ways to get "Account Verified" (the other ways being to have payment info on file with either secondlife.com or XStreetSL). It is supposed to prove that you're an adult, so I don't think it is available for members of Teen Second Life at all. Age Verification might ask for different legal documents for different nationalities/countries of residence, but I don't think it would ask for a mobile phone number, either.
If you ended up in the "wrong" variant of Second Life due to giving incorrect data at subscription, either contact support to get the data rectified and yourself moved to the right variant of Second Life, or—if you don't mind loosing your current SL name, L$ balance and inventory—create a new account with the correct data and cancel your current one. If you ended up on the "right" variant of Second Life although you've given wrong data, you might still want to get your data rectified, because e.g. the given date of birth will determine when a Teen Second Life Resident will be moved to the "Main Grid" and because you might be banned for breaking the ToS. (Unlikely as long as you don't give them another reason, but it can happen.)"