The Collaborative International Dictionary of English v.0.48:

youth-on-age \youth`-on-age"\ n. (Bot.)
   A vigorous perennial herb (Tolmiea menziesii) with flowers
   in erect racemes and having young plants develop at the
   junction of a leaf blade and the leafstalk.

   Syn: pickaback plant, piggyback plant.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

youth-on-age
    n 1: vigorous perennial herb with flowers in erect racemes and
         having young plants develop at the junction of a leaf blade
         and the leafstalk [syn: pickaback plant, piggyback
         plant, youth-on-age, Tolmiea menziesii]