Age not important but must have enough bulk to permit the taking of cuttings.
The cutting should be of firm new growth. apply 0.3% IBA (Rooting hormone) stick in well drained soil & mist taking 28-75 days to root.
can also use seed or division.

When cuttings take this long to root, can they harden off in fall properly? It will be late summer by the time they're rooted!

I've tried taking clematis cuttings but I don't have the patience to mist or bag them and thus I haven't had success with that method. However, soil layering has worked everytime for me.

Mine start themselves, from a fall blooming clematis. where it touches the ground, it roots. So I think layering would work great.

This is good to know!! I will try layering mine
thx!
Karen

what do you mean by layering?