Well it doesn't need to be a young person per se. For example like you break your limb during limb lengthening, for regeneration you would quite literally cut your leg off (in a controlled setting of course), and then regenerate it but longer so you end up taller. You can theoretically do this to all limbs, and if technology is advanced enough maybe to other parts of your body too. You're not changing your genes or anything.
And even humans can regenerate. For example kids under 5 can regenerate their fingertips, so if it gets cut off it grows back. We just need to find out how to do it in adults. Livers in adults can regenerate too, to an extent. If you cut off part of your liver it will grow back.