Speaking as someone you'd probably consider an "old lady..."
Some are just spiteful biddies who have been miserable all their lives and can't stand to see anyone happy.
Some really are just bored, too lazy or stupid to do anything useful, so they snoop and snitch and gossip.
Some are control freaks and get furious when their family and acquaintances don't do what they tell them to do.
Some are genuinely worried about younger people--grandkids, for instance--and are trying to find some way to get through to the kids. Because even though nobody under 20 thinks anyone over 50 EVER felt SO in LOOOOVE.... anybody who produced kids probably did, at some point, feel that way, and a grandma who had to get married to a jerk because she was pregnant does not want to see her granddaughter make the same mistake.
Also, I suspect gossip is hardwired into the human genome. Gossip can have a useful function. In small towns, or even further back, in tribal groups, the elder women were the ones who kept the knowledge that needed to be passed along--how to clean wild game so the meat didn't taste bad, how to prepare food so it didn't make you sick, what herbs were good medicine... and if, for example, someone in the town was getting drunk every night or beating his wife, or a woman was starting to spend waaay too much time visiting a neighbor instead of watching her kids... gossip was an indirect way of expressing community disapproval and pressuring the offenders to get back to their responsibilities.
And you can't tell me old ladies are the only ones to do it. You just listen to a bunch of old guys sitting around... except men call it 'exchanging information.'