I recently went to do my welcome back
But I was told I wasn't up to 18
I should go back

Hmmm, are you the one that bought the SIM in the first place? If yes, go back from where you bought it from and see if they can help. However, the alternative way is to ask you senior siblings to register the SIM in their name.

pls i lost my mtn sim and pls i need it back but i already lost the5 sim pack how can i get the sim again

Visit Nearest MTN office for help, that is the only way, you can read more about MTN welcome back here > https://www.xtremeloaded.com/13633/requirements-for-mtn-welcome-back-sim-swap-and-new-sim-buying-2021