Am I able to use 16+ Oyster Card when I am 18?

I can't imagine why not, make sure you carry your birth certificate with you, but a photo i.d. of you and your age would be better and can be easily done by going to one of those photo booths, and getting an affadavid saying that this is a true image of this person and get it certified by a jp like myself or someone who knows you well, after all it is a legal doent, so jps, or pharmacists, doctors etc., that know you, all usually qualify, hope this helps you, goodluck :)