Welcome to Stamp Collecting Forum!
In response to your specific questions, the 7-9 year old range is usually good for starters.
As many on this forum will attest to (myself included) there are too many other things to get kids attention in their teen years, so very often the stamp collecting hobby is put aside for awhile, but in later years many turn back to it.
Areas to collect depend on ones personal interests. If one has interest in the US start there; if international events interest them start there; if a particular topic is more interesting, you could start there, too. The beauty of stamp collecting is there are no right or wrong answers and no right or wrong way to collect, it just has to keep the interest going of the collector and the rest usually falls into place.
Take a look at some of the responses and resources cited on this previous thread that deals with a similar subject. You may find some helpful information and links with puzzles, games and lesson plans for your own use:
https://goscf.com/t/17635&whichpage...Terms=schoolAnother good link put out by the US Postal Service is available here:
http://uspsconnection.com/stamp-scoop/I hope this is helpful.