Oldtimr did a great job of expanding on issues related to "how far". I started bow hunting in a serious manner in about 1962-63. I switched to a cross bow several
years back as many folks my age do because of injury, drop off in upper body strength, unsteadiness, etc. For sure using a cross bow as opposed to a compound, recurve or stick is no magic pill when it comes to being a lethal shot. If you get the right set-up as far as bolt and broadhead, and practice a bunch, you should be good to go at 40. In fact you should be unable to shoot at the same bull for fear of damaging bolts becasue your groups are so tight. And don't shy away from using a relatively heavy broadhead. For sure 125 gr. broadhead weight is of no disadvanage as far as I can tell. Weight forward on these shorter "arrows" has really proven to be a big advantage to my accuracy.
The big deal as I see it is that plays into distance as well is that one does not have to execute the drawing motion and then hold that draw weight .... and the rifle like weapon is fairly easy to hold very steady. For me, at least, these factors put me back in the tree stand for archery seasons.
I found it relatively easy to become a good shot with my cross bow, but make no mistake, I do a heck of a lot of practicing. Not as much as I did when I shot my compounds or recurves, but I still practice a good bit. I play around with 50-60-70 yard shots when I am piddling but I still stay around 40 yards as a max for trying to take deer. I did try a shot of about 75 yards at a coyote. Scared him but that's about all ..... over his back a few inches.