I was over 50 when I got my 40 acres. I did a LOT of clearing of brush using a machete: I was able to slash for hours at a time, switching between hands; got to be pretty good. Now, the reason for this approach is that I had no idea what was underneath that brush, or, for that matter, where I was going (only in a crude sense): lots of areas machinery couldn't go anyway- see my comment below about getting an excavator stuck!). Oh yeah, my wife would also be out swinging a machete (and she's just a tad bit older than I, not to mention diminutive in stature).
Had I went about hiring out for clearing it would have meant that I'd have had to have a plan! I was aware long before acquiring this property that one very good bit of advice is to live with/on the land for a full set of seasons before doing anything major. My slow clearing has allowed me to make better decisions about how to make my property work. I've gotten 5 ton excavators stuck, and I knew full well where I was (oddly enough my smaller, wheeled tractor had just been in that area and I didn't get stuck); someone not familiar might do a LOT worse. One other thing which matters in my case, in my area of the world, is that I have to be mindful of not making huge, rapid changes such that they'd draw any unwanted attention: I'm good with my neighbors, so no worries there. Anyway...
After doing some research on a bush hog for my newest tractor I got to understand a little bit more about them. There are bush hogs for CUTs as well as UTs that are able to cut up to 4" diameter trees. Well, we know that they can cut thicker, it's just that that's what they're rated to do and keep doing it for a long time. I cannot fathom chewing up stuff like that, but there it is. Bigger stuff can be cut with, yes, a chainsaw (I didn't fall it [it was too close to the house so I hired it out], but I did recently buck and cut up a cottonwood that was 44" at the butt; I've taken down and cut up 3' diameter stuff): even my wife can run a chainsaw- knock on wood, we've been at this for some six years now w/o incident. Up until just getting my new tractor and a used 6' Bush Hog 286 (good for 3" stuff- it's been fortified) I'd managed to deal with things using my
B7800 and a 5' "light duty" rotary cutter: biggest gain was when I got a tooth bar! If you cut up the tree trunk a ways you can push over a lot of stuff. I've got various clumps of stumps and other large wood debris to get out of my way, so I've purchased a grapple (only possible with my newest tractor).
On the infrequent occasion I need to remove BIG stumps (I will bury them, most of them- have one that was so massive that I'd have had to dig to China!) I'll rent an excavator.
I/we live here, so having [most of the] equipment and time makes it a bit more convenient.
I don't ever tell people what they should or shouldn't do (unless they're trespassing :laughing

. I'm just providing info on what I've done as a way of musing. Every situation is different.