I cut a bunch of BL saplings from my wife's Grandparents property a couple springs ago.
All are 8-13 years old, and Pats right, all have 3 rings of sapwood, a couple have 4.
I recently made a bow from one, overbuilt, took very little set, shoots awesome, but is covered in frets from tip to tip haha. If I were to grow it, I'd probably wait till spring, and go pull out some smaller saps and transplant them.
I'm started 3 Osage trees 6 months ago, I'm 33 so I hope I can possibly make a bow from them one day!