As per legal point of view, she must complete 18 years to marry, but she is still in 17th. If you approacha a "Qazi" he also demand to submit 'documentary proof'/Certificate about her Date of Birth or Age.
" Dulha Dulhan Razi to Kiya karega Qazi ". If you are serious then you can approach to Qazi of your location, may be he can help you.
But Mr. Asif, as you are saying that,
" we both are muslim" ...... " her parents know everything about our relationship and also about physical relation....." ........ " they will going to claim of rape on me"
don't you know what is 'ISLAMIC' and "Quranic' teachings against marriage and pre-marital relationship...... It is "ZINA" if you both indulge in any physical relationship before to marriage......" Zina is a Big Sin ( Gunah-e-Kabira)" .
As you are further saying that you are still in 22, a teenage,
If you marry at this Age........ what is your standing ...future after marriage...how can you both lead your life ?? .... think once again.........
If possible inform all this facts to your parents also and ask them to interfere and have a talk with parents of girl and try to postpone the matter either till she complete the age for marriage or till your standing for future life.