Age is not the factor of concern. A whole lot of us in our 70's ride a variety of bikes - in my case, 2 road bikes, a mtn bike, a recumbent,

The other factors are of more concern - $$, physical condition, where you want to ride and why, gearing.

Generally, even with a triple on the road bike, your road bike gearing is not going to be nearly as low (easy to pedal up a hill) as a mtn bike, which is geared VERY low. Of course, adjustments can be made, but, eventually you would not really have a road bike, IMHO.

Since you can not stand, this may be of concern, although I have no knee problems, and never stand while pedaling, except to relieve pressure on my butt.

I sure would try some out - and - your experience will be somewhat different (and likely better) than your 10 speed experiences, and a whole lot different from the 29".