You know, these past 2 or 3 seasons, I've read so many articles from folks saying that the actor's (and character's) have gotten too old and have aged and arent' "cute" anymore. I'm sorry, but to me, MITM was never about the "cuteness" of the boys. Sure, they've aged but families just don't stop when the children get older. They may branch off as some of the kids go to college and begin their lives but the family connection, as I think was pointed out very clearly many times this season, remains strong as ever, especially with this family. No matter how far they may be from one another, they're still a family and they'll always have each other.
In my opinion, a spin-off, while probably not lucrative enough for the studio to finance, would have been a great idea. We could have followed one or two characters as they continued on with their lives outside of the home. They'd always have their family to fall back on and share their experiences with and if nothing else, they'd have taken lessons learned, sometimes the hard way, which would have prepared them for situations they met in the future.
Yes, this show was about childhood and growing up, but I think if you ask any parent, they'd say their child will always be a child in their eyes, whether they are 8, 15, 25, or 40 years old and I also bet they'd say they think there's still quite a bit of "growing up" yet to be done in their child, no matter their age.