juvenescence (n.)

1759, "becoming young, growing young in appearance," from Latin iuvenescentem (nominative iuvenescens), present participle of iuvenescere "to grow into youth, grow young again, regain youth," from iuvenis "young man" (see young (adj.)). Its use in a sense of "immature, undeveloped" (by 1875) is etymologically incorrect.

see -ance.