Junipers not so easily, I have had budding on 5 years old wood, but that might be an odd occurance. Older than that will never backbud.
Had a young juniper in the garden, when there was undergrowth from otherplants that caused the lowerpart of foilage to die. When i removed the undergrowth, within 2months there where new buds at the trunk. That juniper is also about 5years old now. Never did anything with the roots, also did never prune it. As it is just for the garden. But i was surprised to see the backbuds, as i did not expect it to do that.
Junipers do backbud, albeit not reliably nor predictably. I have 45 year old junipers that have popped buds on the trunk. There is no maximum age at which bud can appear.
If you grow a juniper, typically one leaves lower branches kept short OR you let one or two branches run long so the trunk fattens up, and later you cut off the sacrifica branches (remember the option to airlayer) and create a short yong from the leftover stumps.