Pinched Harmonics

I havent changed my guitar strings for a long time. Can this prevent me from playing good pinched harmonics?

yep, it will also detract form your overall tone. If the top (not-wound) strings look burnt after you clean them, it is time to change 'em.

I change mine, well it depends on how much I have been playing, but my main axe gets a new set every 1 or 2 months.

Giving 'em a good wipe-down with a soft cloth after you have been playing is another good idea.

Strings that have grown beards are never as good sounding as pristine new ones !