Imo the age of the plant is not really the relevant issue. It's the age of the growth you use. If you prune the older plant new growth will ensue. If you're doing softwood cuttings the growth will be this year's, so no older than similar growth on a young plant.

But personally I would use semi ripe cuttings, ie this year's growth taken in late summer, not soft cuttings.

Thanks! I heard from 2 experienced nurserymen that cutting success rates are low with serratifolia and even lower with older plants. I'm not using the softest tips now but using the firmer semi-ripe new growth 2 or three leaves down the stem. Later in the summer I'll use the tip sections that are semi-ripe. Any thoughts on media and container size?

click HERE for my previous post about this plant. I did include one pic with a rooted cutting. Easily rooted, I have intermittant mist. Not a fussy plant.

I see Michael Dirr is promoting a recently discovered cultivar with red new growth but don't know if it is in the trade yet.. The older P. serratifolia I'm going to attempt to clone is not like Green Giant or any other serratifolia cultivar that I've seen. With sun they normally grow into trees and are covered with heavy spring blooms.. This specimen is 50+yrs old and approx. 15-20 ft tall X 15-20 ft wide, has very dark green, drooping, markedly serrated leaves, the leaves are slightly larger than other examples at 4-7inches, flowering is sparse, . The new growth flush is impressive, lighter green leaves emerge with red stems, there is the occasional red leaf. (see picture below) Granted it is growing under mature willow oaks in open light-shade that may reduce flowering but gets more than enough sun to show it's full growth potential. I suspect it may be a new variety, possibly a multi-stem dwarf as it apparently never tried to take on a 'tree form and hasn't exceeded 20 ft in more than 50 years. Also it has proven disease resistance as a short distance away an entire planting of P. fraseri were wiped out by leaf spot. I hope the propagation goes as smoothly as you suggest,