The absolute lowest a 4th-level PC in this particular game could have would be 13, yes - but they really have to TRY to get a defense that low. And yes, I do think it's much more common for a PC to have at least a 12 in each pair of defense abilities (STR/CON, DEX/INT, WIS/CHA) but sure, a PC can put 10/10 or 10/8 or something like that in a pair of those stats (though I think that's a bad idea). I wouldn't say "You could EASILY see..." but yes, you CAN see it.
If you use the auto-build feature in the character builder, you'll generally have such a defense score. If you follow the building advice in the books (i.e. mentioning primary and secondary scores), you'll quite likely arrive at such a score. In fact, if you crunch the numbers, you'll find it's probably numerically the strongest option to have a weakness, though that depends on how you define "strongest" and what it is you look for in a character. Some classes have aligned primary+secondary stats, making it prohibitively expensive to avoid such low nads. I wouldn't expect everyone to have a 13 by fourth level, but I'd expect every party to have one. Cautious players that prefer to have all bases covered might well choose a higher tertiary stat; the risk-takers will probably prefer to have a more powerful strengths at the cost of more pronounced weaknesses. Also, note that in the computation to arrive at 17 as the mean number in the OP's scenario, I (optimistically) assumed a defense-focused build, with a stat distribution for maximized defenses. All in all, it doesn't actually help very much since the extra point-buy points reduce the primary or secondary score which also affects a NAD.
22 AC is on the high side EVEN FOR A DEFENDER in this game.
The spread of defenses in 4e is rather narrow. For defenders, you'll find that most defenders (as their name implies) have good defenses. On this side of the fence that implies a
minimum of AC 22 - you'd have to actively try to get lower than this. The other type of defender prefers offense to defense; such a defender forgoes a shield for greater offense. The only exception to this rule I can think of is the warden; and this is an unusual class in that it has more hitpoints than any other class in the game, at the cost of lower-than-normal defenses (and even they manage AC21 virtually automatically, and AC22 if they want to, assuming a build with a shield). If you look at the various defenders in the game, you'll find that the defensive defenders (more common) all have scale mail+heavy shield proficiency or better, and that the corresponding offensive build loses about 2AC.
If the absolute maximum AC a 4th-level PC could have in this game is a 23, I would call 22 "good for a defender" rather than "average for a defender". And of course for a striker (as in this PC's case) it's very high.
A Swordmage starting with a 17 or 18 primary stat and the Improved Swordmage Warding or Hide Armor proficiency feats has AC24 (and I'd expect pretty much every swordmage to get either feat eventually). That's without magic items; which in most games would enter play and could contribute to AC. You could search the charop boards, but I'd bet you'll find builds exceeding 24AC with level-appropriate gear. E.g. the high-AC rogue might trivially have a Mage's Parrying dagger for a very easy +1AC (coupled with feats for potentially more).
So you'd expect only a few characters in a game to have this kind of AC (not everyone plays a defender or invests feats and other choices to reach defender-like AC), but you'd expect most parties to have such characters; unless the only defender chooses to focus on offense, and no other PC chooses to focus on defense.
The OP's distribution is again entirely normal, and the value of 22AC isn't particularly high - it's bog-standard defender AC.
Edited to add: It's also worth noting that AC's just don't vary that much. So 23 is high yet 22 is normal for a defender; that's because in 4e these things hardly diverge.
