1. the supreme court has opined in Indira Sarma case that live in relationship is an individual contract between the parties to live together.
2. they can separate their life any time with their consent.
3. so the legal point became settled. you can't live together in live in relationship because she is below the age of 18 years because she cannot give valid consent. but when her parent are agreed then you can live with her because there is valid consent from their parents.
4. after lapse of 11 months you can solemnise marriage.