I would say maybe 8 or 9 years old. Honestly, I used to be a tutor and most children before that age group seemed a bit too young to be toting an expensive piece of hardware around. When you consider the fact that most kids under the age of 10 are under constant supervision, you may want to wait a bit longer than 8 or 9! Maybe instead of placing an exact age on the situation you can see if their mentally ready: https://ph.theasianparent.com/5-questions-ask-kids-giving-phones/
I think it's not necessary at all when the kid is too young. What you would want to do is to keep a better eye on your kid, or ask him/her to memorize all the pertinent information whenever it will be needed, like telephone number and address. While cellphones can be a great tool for communication, it can also be a bait especially for unscrupulous persons. Personally, I won't give my child his own phone probably until he finishes elementary.
I think it would depend on how soon your kid would be able to handle the responsibility since cellphones these days are powerful communication tools. Make sure that your child knows to use the phone only for emergencies. That way, they will also be safe from scammers sending fake texts etc.