I like the idea of defining "involvement." Perhaps date of last purchase would be a factor.
For example, my dad could technically be "involved" in the hobby because he has HO trains that we had when I was a kid. I don't think they've been out of the box in many years. Same with most of my N scale stuff.
If you raided mom and dad's house tonight, you would find evidence of involvement in model railroading. However, it would not be active involvement. 
