I have a fire mage at 20, 39, 49, 70, and 80. I don't really enjoy fire at 20 or 39 very much; you're just missing too much of your toolkit to really be fun to play or effective, and missile spam is just orders of magnitude better. It's not any fun, but it's better. 49 is the earliest bracket I'd play fire in--gearing isn't as bad as you might think, especially if you're on a populated realm, and BOAs and dungeon drops will make up a lot of what you use. Here are my 49 fire mages:
https://worldofwarcraft.com/en-us/character/skullcrusher/Tzvetan
https://worldofwarcraft.com/en-us/character/velen/Vladrinor
One of the things that makes 49s really fun is that you have a lot of very viable, competitive gearing/talent set ups. You can stack SP and run Tinder as your pvp talent with firestarter as your level 15 and burst people to 40-50% in your opener; you can stack haste and run greater pyro as your pvp talent with firestarter as your level 15 and have an even stronger, albeit much longer to cast, opener; or you can just go for as much crit as you can get. All three are fun and viable; I tend to favor option number one, but it's really down to personal preference.
Things get a bit more homogeneous later on, simply because the 5 int/5 crit gem is so much better than anything else, and the variety in competitive gear shrinks way down. After 49, you'll basically be trying to get as many gem slots as you can, which takes away the variety.
I haven't enjoyed 70 that much lately, even though I've spent tons of time/gold on my 70 mage; there's just too much of a difference between 70 and 79, IMO, and I'm not interested in leveling my character :/
My 80 mage, on the other hand, is still a work in progress and is an absolute blast to play, and is very competitive, going toe-to-toe with 85s, 87s, and 89s without much problem, even though he's still missing a lot of gear. Here he is:
https://worldofwarcraft.com/en-us/character/velen/Virzaan (again--still working on gearing him).
All that said, if you really want to be a godlike, world-destroying fire mage, I'd probably recommend 59s. I don't play them, but my buddies do, and you can get an insanely high crit rating and SP due to BC dungeons and all the gem gear, which you'll naturally fill with the 5 int/5 crit PvP gems.
So for fun and variety: 49. For four available expansions worth of dungeons, plus some really fun scaled pieces and arguably the most balanced bracket, 80-89. To melt people really fast (but also be subject to turbomelting from other twinks), 59s.
GLHF.