Well, I've had two comments so far - I will answer the easier one first - there are many fine teachers for all voice types in Milwaukee, but in this setting in particular, I prefer to not make recommendations for something such as that.
The other question is a little tougher - what is the maximum age to undertake a singing career - any time one makes a blanket statement about something like that, many exceptions to the rule pop up. If you start later than your 20's, you will probably not find many young artist programs that will be available to you. In addition, big competitions like the Met Auditions, have age limits that top out in the low 30s. So, if you do start late you are going to be in a position of creating your own opportunities, and that can be difficult. I will give this some more thought, and comment further in another post.