I have bought bonds for my daughter using her SS# as co-owner with me, and visa versa. Have not cashed any in, but I don't see a problem with the age of the child. I believe the tax would be paid for by either your child (still minor, or now major) or you. No different when you get a gift bond that has the givers ss# - they wouldn't pay the tax, you would. Certainly a minor child's tax burden is probably less therefore less tax. I started doing this, thinking of it in case my daughter's college doesn't take credit cards, and if they do, I'll have it for retirement. So, unless the rates go up (some I bought were much higher than now), I won't cash them in for awhile.
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Ms.DtG