Question about my first batch

Just tasted my first batch which is a Northern Brewer Irish Red Ale Extract Kit. There is what I think is definately the "green" taste to the beer. I let it sit in the primary for about a week, transfered it to the secondary for about two and a half weeks, kegged it, put about 20 PSI in it, disconnected the co2, let it sit for two days in the keezer, reconnected the co2, filled it back up to the 20 PSI (had dropped down to about 10 psi), let it sit for a few hours (couldn't wait anymore at that point), dropped the pressure to about 7 psi, poured a glass, had a very grassy/leafy aftertaste. From everything that I've read on this forum (which has been very helpful so far),this is because the beer has not aged enough. My question is if this it true (grassy taste because it needs to age a little more), and if so, do I need to put the original 20 psi back on it while it ages or the 7 psi serving pressure? The carbonation seemed to be just about right (maybe a little weak but that's being picky). Also, how much longer do you think it needs to age?

first off, i think you should let it sit on the primary yeast cake longer next time and skip the secondary. especially since you're kegging, just crash cool the fermenter in the fridge a few days before kegging and it will clear up a bunch. second, there's really no reason to disconnect the CO2 after purging the head space unless you're going to age it for a while. normally, i just leave the gauge at 30 psi for 2 days, then reset to 12 (that's the sweet spot for my setup), and after a few days at 12, the carbonation is usually about right. so, i'd be surprised if you have enough carbonation in your beer at this point. i would just hook the co2 back up, set it to your serving pressure and let it sit for at least another week, you'll be surprised how much the flavor will change with the coming weeks. and i fully understand not being able to wait, definitely try a glass here and there to see how it changes, it can be very educational.

Yeah, I wanted to leave it in the primary longer but I got a deal on a secondary so I have a total of two (one primary and one secondary in the original kit that I bought), and I wanted to free up the primary to brew another batch. Going to get another primary in a few weeks but until then I think I'm going to keep using the secondaries so I can really get the beer flowing!
So it should probably take at least another week for the flavor to change? How am I going to wait that long?!?!

Also, keep in mind that beer ages faster at room temperature. At fridge temperatures, the aging is slowed down considerably. So, I'd actually keep the keg at room temperature another week or two, then chill it. That's hard to do- but it'll take away any "young" flavors.

I've never thought of grassy flavor as young beer- to me that's more of a hop flavor. It'll still fade quite a bit with some aging. Green apple is definitely a young beer flavor, as is yeasty.

It really doesn't seem like a hoppy taste, I guess it does have a very faint sour taste to it which would kind of be like a green apple. Not really sure what a yeasty flavor would be, would it kind of be like a doughy taste to the beer?

Green Apple (and your schedule )would definitely suggest the beer is still young yet. As Yooper said, it will age faster if not refrigerated. Personally, I like to do final conditioning/storage at around 50 degrees.

Also, as has been suggested, give your beer more time in primary next time - I find two weeks is often adequate when doing a secondary (obligatory use-your-hydrometer statement).

Personally, I almost always do a secondary, even if for just a couple of weeks. I tend to do more lagers, so a secondary is pretty much a given for long(er) term conditioning, but I also like a secondary, particularly with kegging, as it helps knock even more yeast, etc, out of suspension. Beer is clear pretty much from first draw to last, and it's nice to not have to clean out a layer of gunk from the bottom of your corny. Perhaps most importantly: there's nothing sadder than kicking a keg, and having the last fine, sweet glass of brew turn to mud when the faucet hacks a trub loogie in it.