Like I stated in an earlier post,if an Amish girl wants to date someone that is not Amish she will either have to leave the Amish or that person would have to join the Amish. Inter-lifestyle households do not exist among any Amish communities. The reason this is not allowed is it could cause too much outside influence when having daily interaction with outsiders.
In the community that I grew up in the rule was that you cannot start to date until you are 16 1/2 years old. In general I think it is 16 year old but also some of the more conservative communities say not until 17 years old. That rule will vary from community to community.
After you are old enough to date then you are also old enough to become a true member of the Amish church by going to classes for instructions on what it takes to truly live the Amish way and then be baptized. Baptism has to occur before you can get married. For the most part Amish women get married between 17 and 25 years old.
I hope this is helpful and if you have more questions please submit at www.TheAmishLady.com