Just about anyone can do it safely, with the exception of people with very specific health situations. Even very skinny people who need to but cannot gain weight seem to reverse that trend following a prolonged fast. The body is very smart in turning a prolonged fast into a series of corrective actions needed to maintain its own well being.
For two years prior to this fast I suffered from mild pain in my left knee which made it very difficult to get up from the floor or run for more than a few minutes. Two weeks into the fast, that knee began to hurt like crazy. By day 16 I was unable to bend it. But that was somewhat expected. Many peopl who undertake a prolonged fast report what is referred to as a "healing crisis"- when the body is in a state of self healing during which pre-existing symptoms flare up as the body works to get rid of them.
Who knows what I had in my knee. Gout, pre arthritis, some nasty build up of fluids... Either way, it's all gone. After 3 days of being unable to stand or even slightly bend that knee - it all disappeared. I woke up on day 19 and it was as if I had a new knee. All symptoms are gone since then and I have resumed my hiking and biking activities and am glad I let it heal itself.
And to answer your other question, at 5'9 I weighted 182 at the beginning of the fast and 147 on day 23. I wasn't terribly overweight, and now at my ideal weight.
And day 21 is far from being at the point of true starvation. That doesn't normally happen until at least day 45.