Ask Dan & Jennifer

depends on who you are, what your relationship with them is, and what are you expecting out of it. if you dont want to, dont. if they are online, they wont hurt you

A couple of things I'm curious about. How old you are is the first thing. You say "older guys"... to a 15 year old 20 year old guys are older. But to a 25 year old, 50 year old men are older guys... so, I just want a point of reference here.
Also, where are they coming from? Are you going into chat rooms, are you talking myspace or facebook kind of things, or are they looking your profile up... basically, how are these older guys contacting you.

Finally, I'll give my advise. If you want to flirt with them, go ahead, but as always be cautious. You never really know who you're talking to.
But, if on the other hand, you don't want to talk to them, then tell them so. There's no law written that says you have to talk to every person who wants to chat online. You can tell them sorry but you're just not interested, you can tell them to get lost, you can ignore them, you can block them. It really is up to you, and how aggressive they are being. If they continually harass you, you can report them to the internet provider or to whomever hosts the place they are contacting you at... or possibly the police if it is severe enough. But, thankfully, usually just telling them no and then ignoring them works.

Good luck!