Ive been trying to play Blink 182 - What's my age again, i can play all the chords in a down stroke motion but i can't strum the strings up and down without hitting the other strings, ive looked at videos that show how to play it but they make the srumming look so easy! Can anyone give me some advice? Cause ive been practising this song for weeks and still i can't do it.
Bare the other strings with your finger. Like if you're playing A5 play it like this

e x
b x
g x
d 7
a 7
e 5
Just practice. Eventually you will get used to it and won't have any trouble with it.
You gotta mute the other strings, it takes effort at first but soon you'll do it without noticing. Just make sure the only strings you are holding down are the ones you are playing.

e x
B x
G x
D 7
A 7
E 5

For power chords on this string you can mute the top three strings by laying your fingers flat across them all, but not holding them down.

e x
B x
G 7
D 7
A 5
E x

To play power chords on this string you gotta do the same but mute the low E string with the tipo of your first finger. Any help?
try muting the strings with your ring finger or pinky (which ever one is on the higher note) by arching that finger just enough so that it mutes instead of ringing. Also, if your root note is one the 5th string or whatever, mute any higher strings by resting your pointer finger on it. It takes practice though.
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