This is the Smarty community forum, we are customers like you, we don’t work for Smarty.
You usually need your own credit card to prove you are over 18, as people under the age of 18 can’t get credit cards.
However, if you don’t have your own credit card, get in touch with Smarty directly via web chat where they will of course just need to confirm you're over 18, usually through receipt of valid photo ID such as a Passport or Driving Licence. If you own no ID, then, as I don’t work for Smarty, with this being the afore mentioned community forum, there may be some other way to verify your id, but off the top of my head, I can’t think of one.
Web chat is available 8am-8pm, 7 days a week. Just look for the green chat icon on the Smarty Mobile website - https://smarty.co.uk/
