I took up cycling about 9 years ago at 61 and started out doing road and cross country (XC). I built up where I was alternating metric centuries with 18 mile single-track rides. Road got really boring for me
so now I'm 99% XC/single track and have been for 4 or 5 years. I like the extra challenge, love the technical stuff especially on the steep climbs and being in the woods is so much better than the roads.
It's also a more complete total body work out as well.

I spend about 6-months per year riding in N Florida and the other 6-months in the mountains of N Georgia and SE Tennessee. I love the climbs.

For one starting out it's important to develop a good skill set quickly. No need to reinvent technique and the better one is the more fun it is plus the fewer times you'll be hugging the ground.

I learned my skill set from the video Performance Mountain Biking, the Basics and Beyond by Ned Overend. His book is very good as well. You can get both at Amazon. At first I'd watch the video, then go ride and watch parts of it again to see if I was doing it right. Now I watch it about once a year to 18 months as refresher training. That helps me keep my speed up.

While "Overend" may not be the best connotation for safe mountain biking, Ned is famous for winning six national XC championships and the first worlds XC in Durango at a relatively old age of like his late 30's.
He's also famous for not getting injured, so he's my kind of guy as I like to push the envelope on the trails.

Al