There is always the debate about what is the most effective strategy for reaching new people, change an existing church or simply start a new one with its own unique culture. In my opinion both have their strengths and weaknesses, so the decision should depend upon the context.
There are several things I have noticed when churches are started with one particular age group in mind as the target. All of your programming, especially your worship can be one style depending upon the age group in the room. The staff and the facilities can be designed with the needs of this age group in mind as well.
Although this may work for a short period of time there eventually will be serious challenges that must be addressed. One day all of these single adults will get married and then they are going to want programming for their children. Eventually, this same group will become empty nesters and that will bring on another whole set of needs.
My point is simply this, every church given enough time will become multi-generational unless you are going to tell people at some point you need to leave. Since that is a reality then why not start with a multi-generational model from day one that will avoid all of these potential crisis points that can kill momentum and destroy unity.
An even more significant reason is that it is biblical. Every person is important to God from the youngest newborn to the oldest senior adult. The gospel will always be more receptive with younger people but the ministry of the church must include the widow who is all alone.