".............Does anyone know the science behind why cuttings from young
plants are more likely to root sucessfully? ........."

I would have thought that the question should be why in general cuttings
taken from soft or semi ripe stems roots better than those taken from
ripe/mature stems.

The "Skin" is thinner and the cell activity is greater with young growth
than more mature cuttings with a "Dead" laver on the outside, this is one
reason why we score down the stem of cuttings, to expose the cambium layer
and to promote the formation of calluses and so the root formation.
If the growth is to soft and you have a slow rooting species then the stems
are liable to rot before rooting has taken place.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk