How fast can I get? This is a regular question posed on running forums. My answer to that is something to this effect: That's for you to find out by training consistently. I don't have a crystal ball and it's difficult to predict how your body will develop and respond to a steady training regimen. The good news is that your maximum potential is almost always far greater than your starting point. Sometimes, it takes time for the talent to come out so don't be discouraged if you are very slow when starting out. Many of these slow starters turn out to be pretty good runners if they stick with it. For the record, I've dropped from 1:59-1:28 in the half marathon and 24:55-19:13 in the 5K. My circumstances are not typical but the progress that I have made since starting my 2nd running life is not unheard of for a beginning runner. Some have greater distance between their floor and ceiling than others. For example, an ex-athlete who stopped exercising for a decade will likely improve faster with the same training than a person who has always been physically active but never trained formally for running.
In my many years as a runner, I've seen folks who start out very slow and end up faster than I am. On the other hand, I've met just as many who are pretty good from the get go but never get much better. This phenomenon can be best explained by their training especially as they get older. It's not too unusual for a 13 year old beginner with loads of natural talent to run a Mile under 5:30 or a 5K near 20:00 despite never really training for it. However, if this person seldom trains, that great Jr. High time that they ran will stay the same in high school then begin to get worse after the age of 20. I know a guy in town who claims to have run in the 17s for 5K in high school. Two decades later, he still jumped into the occasional local race despite minimal training. He's now 30 pounds overweight and gets slower every year. His last 5K was over 27 minutes (3 minutes per mile slower than his PR). At the other end of spectrum, I've read about some runners who allowed themselves to balloon up to near 300 lbs yet the hidden running talent is still there. Indeed I've read some inspirational stories about these types who go on to run low-3 marathons.
A few of the Gnomes have introduced their young sons to running. One of them is only 9 years old and can already run a 21:30 5K. What is the best age to start? I'd say it's fine to introduce your child to running around 8-10 years of age but I recommend no real intense or structured training until about 13. The younger a person starts, the closer they can get to their true ceiling. A 20 year old beginner can improve until the mid-30s especially at longer distances. 30s and 40s year old beginners generally have about 10 years of potential improvement before the headwind of aging wipes out the gains in fitness. That said, you're never too old to start. Even a 60 year old can usually improve for about 5 years.
Everyone has their own limit to how fast they can go. Few people will reach it. The general consensus is to reach your limit, you must train at 60 MPW with at least one quality session in addition to a long run and sustain it for several years. Very few people are willing and able to put in that type of work. I'd rather sacrifice a few seconds per mile than risk major injury, burnout or strained relationships. I've already got the half goal accomplished, the 5K tied and I'm so very close to the rest of them. Once again, anything beyond the stated lifetime goals is a bonus. There will be no obsession with a sub-18:30 5K or 1:25 half marathon.