Methods:
Tapering-
You use different size tapers to gradually (about every 2-3 months) stretch your piercing. You insert the taper through the piercing's hole and it will stretch the piercing up 1 size (ALWAYS go 1 size at a time- skipping gauges is very dangerous).
Tape Wrapping Method-
Wrap 1-2 wraps of PTFE Tape (Plumber's Tape- it's non-porous and won't harbor bacteria) around your jewelry every 3-4 days to very gradually stretch up in size.
This is a much healthier, but much slower way of stretching.
It lowers the risk of blow outs and scar tissue which can make stretching larger in the future very hard or even impossible.
Gauges are the different sizes of jewelry/the piercing itself.
You do not gauge your ears, you stretch them (thank you SO much for getting that right! We die a little inside whenever we hear that people have 'gauges' or are 'gauged.').
Smallest to largest- 20g, 18g, 16g, 14g, 12g, 10g, 8g, 6g, 4g, 2g, 0g, 00g, 000g, 7/16", 1/2", etc.
Here's a gauge chart:
http://www.bodyartforms.com/measurement_guide.asp
There are no age restrictions on stretching your piercings and buying the equipment as long as you are doing it yourself. If you go to a piercing/tattoo shop to have a piercer stretch it for you, you will have to be 18, or have your parents with you if you're younger.
Stretching with tapers can be quite expensive over time.
Here's a GREAT site to order body jewelry and tapers from.
They have amazing prices (lowest I've seen), fast shipping, and great quality. I use them all the time.
Tapers:
http://www.bodyartforms.com/products.asp?jewelry=tapers&filter=customyes
You can buy them at stores like Hot Topic, but they're never cheaper than 10 dollars.
Stretching with the Tape Wrapping Method is by far the cheapest method.
EDIT:
Scalpelling lobes is generally used when you're stuck at a size (usually a larger size like 0g+), and you just can't get your ears to stretch any larger by tapering or tape wrapping.
A Body Mod artist makes a slit extending from the piercing's hole- which enlarges the size.
http://wiki.bmezine.com/index.php/Scalpelled_Lobe
EDIT #2:
Forgot to add that Blowouts are when you stretch too fast, and there is too much pressure put on the tissue of the piercing so it twists itself out of the back of the hole (usually the back, but sometimes the front as well).
http://wiki.bmezine.com/index.php/Piercing_Blow-out
EDIT #3:
I can't believe I forgot to add this...
Stretching can very well be permanent. More so at 2g+, but it truly depends on the person. I have 10g piercings that won't shrink at all, but 7/16" ones that will start to shrink up if I leave my plugs out for a few hours. I highly doubt they'd shrink up any smaller than 6g though.
So please keep that in mind before you begin stretching (if you do).