I'm very new to this game and am wondering, If the enemy has say 30 AC with all his equipment and buffs, what would I have to roll to hit him since I have to roll a 20 sided die to determine if I hit or not? Since 20 sided dices obviously don't go to 30 I'm curious as to how this work.
Equipment takes damage under a variety of circumstances, but most of the circumstances are rare unless a character devotes resources to breaking things
Such a character might...
- sunder a item held or worn by a foe. The character picks the item to sunder. If the sunder attempt is successful, determine if the item breaks by consulting Damaging Objects. The Damaging Objects section answers many of your questions about, for example, energy damage versus objects and acid versus leather.
cast a spell that specifically harms objects, such as the spell shout, but unless a spell says it harms objects--either in the heading or in the description--it doesn't harm objects.
Note that some spells--for example, fireball and lightning bolt--readily affect unattended objects (i.e. objects not in a creature's possession), but attended objects (i.e. objects in a creature's possession) aren't usually required to make saving throws separate from the character. Instead, the (usually more narratively important) creature makes the saving throw, and, whether the creature survives the spell or not, the gear the creature possesses remains unharmed.
Also, for example, acid arrow inflicts acid damage to a creature that's the spell's target but that same acid arrow spell doesn't also harm that creature's possessions. Were the caster to target with the spell a piece of gear in the creature's possession, then the piece of gear would take the acid damage instead of the creature.
manipulate the environment--usually via spells like burning gaze but sometimes via items (but, strangely, apparently not alchemist's fire)--so that a foe suffers ongoing damage from an environmental effect. Most things don't do this--and it won't happen unless the thing being used says it does happen--, but among effects that do reference the environment rules, catching on fire is by far the most common, and, so you, know
Those whose clothes or equipment catch fire must make DC 15 Reflex saves for each item. Flammable items that fail take the same amount of damage as the character.
Emphasis on that tedious task mine.
- cast enough spells that require saving throws to force a foe on a saving throw to roll--eventually--a 1. This results in an exposed item suffering harm. If an order is needed as to what attended objects take damage before other attended objects, it's reasonable for the DM to start with the magical attacks chart. Using it for unintended purposes, however, is liable to cause hard feelings. Players really don't like their characters' stuff broken, especially with house rules.
KRyan's laid out well the reasons why the game functions this way, but let me add that even the most realistic of table-top role-playing games rarely track wear and tear on a character's gear.1 That minutiae is better suited for video games or unusual RPGs emphasizing the importance of the characters' gear over the importance of the characters.
- So much so that even Steven Jackson Games's deeply simulationist GURPS, 3rd Edition made tracking damage to a character's shield a wholly optional rule.
There isn't an officially stated lifespan for Changelings (as of writing this answer).
The most important thing to remember is that Unearthed Arcana material is only effectively playtest material. It's not "officially" published or polished and thus isn't nearly fleshed out as much as the core rule books or supplements so you're not likely to find that level of detail until it's actually put into a supplement book, if it even does.
There's only one other mention of changelings that I can find and that relates to doppelganger children in the Monster Manual but even that doesn't give us any information on their lifespan.
However, if you really want something to base it off and get a rough idea then it has been detailed in the 4e Eberron Player's Guide (unfortunately I can't speak for older editions):
Changelings reach maturity at about fifteen years of age, and they live about as long as humans do.