I'm 35 with a 2-year-old so yes I've gone from riding dirt bikes a lot with a little mountain biking to riding mountain bikes a lot with a little dirt biking.
Before my wife and I had our daughter, I was practice riding twice a week and racing enduro on Sundays. Now I'm lucky to race two to three enduros a year and ride two to three times a month.
It's really easy for me to get time on the mountain bike since I have multiple loops out of my driveway that can go from quick 30-1 hour rides to full on 2 and 3 hour rides. I also do a lot of night riding too so it's just easier to get more mountain biking in now. If I want to, I can ride 6 to 8 hours a week before and after work without my wife even noticing.
It's not an either or thing for me and I still see mountain biking as a means to an end... I do it because it's more fun than running and the skills/endurance make me a better motorcyclist, but yes I am doing more mountain biking than dirt biking right now.
I recently sold all my geared parts and bought a Trek/Gary Fisher Rig. It's a singlespeed 29er hardtail. I'm in a lot of pain on my rides but loving the simplicity of no gears.
2009 CRF450X, 2009 CRF250X, 2005 XR650R (street legal), 1999 XR650L, 1986 TLR200 Reflex, 1975 TL250, 1975 TL125, 1983 CR60R, 1981 Z50R