at what age is the right time to have sex

A lot of countries it is illegal to have sex under the age of 16 and you could end up with a criminal record so it's best to check your countries laws on that.

My own opinion on what your asking is that she is still to young  yet and her bodies is still maturing.

 

It is unfortunate when adults decide relationships with children are appropriate. I would suggest letting her date someone her own age would be the right thing to do and the best thing for her well-being.
I would hope you decide that a relationship with a 14 year old is completely unacceptable and if you need convincing that having sex with a child is wrong consider life as a registered sex offender if you get caught.
Please don't take advantage of this girl that should have a chance to grow up without being manipulated into something that she will regret.

The right time to have sex and to engage in sexual activity is when you have hit puberty and when you have PREPARED yourselves. 

An 18 yr old and a 14 yr old can most certainly have a relationship if they so choose. Sometimes the law may not approve of that kind of relationship depending on the country you live in

If you guys really want to have a relationships and you are both agreeable and willing then be discreet, take precautions and treat the girl well. For the guy who is older, please dont hurt the girl. That is my advice to you. Please make sure you dont make the girl pregnant and DO NOT give her STDs. Make sure you look after the girl well and take care of her and protect her. 

I feel this is pretty negative advice. They have already decided to have a relationship. From my experience, if 2 people have decided to have a relationship, then it will happen, irrespective of what anyone advises.
Let them be. Just because the law does not agree with such relationships, does not make it bad or impossible. They will find a way to continue their relationship.
14 BTW isn't too young. 14 yr old girls these days are a lot more aware and mature to handle a sexual relationship even if the guy is many years older.
Right now I think they need to PREPARE themselves with information and the tools to prevent any negative consequences. There is no such thing as being READY. No one can ever be ready but they can be PREPARED.
Let is be positive here and make sure they take precautions and come to no harm.