Adopting a 17 year old, illegal alien

We would like to adopt a 17 year old Mexican boy (illegal alien). We are his legal gaurdians and we have begun the paper work for adoption but have gotten the idea that both his age and his residency status could be a problem. We live in Colorado.1. Is his age a problem? 2. If we do adopt him will his legal status change? 3. Is there a legal alternative to adoption that would allow us to include him on our health insurance and would allow him to stay in Colorado and attend college after high school? One person mentioned Long Term Legal Gaurdianship but I haven't been able to find anything out about this process.Thanks.