As Pete's said, given your situation you need to replace them, although there's a growing argument for keeping some textiles beyond the normal 10yr retirement age plastics don't fair so well.
We've been using both the Boreo and Panga as well as having a few of the Elios kicking about.
The Elios is rubbish! Although they're robust there's just to many fiddly bits for group use so you spend ages fitting the things, and then a buckle pops open, pain in the....
The Boreo's are great in that they've multi-direction impact (i.e. foam all the way around the inside). You can get them as either Club or normal version. The Club version comes with glued in vinyl "sweat pads" so a lot better for group use, where as the alternative version comes with a removable (velcro) fabric forehead and top pads. The Boreo also comes in 2 sizes. The downside with the foam is, although it's more robust than many other foam fillings of helmets, it's still prone to damage, after which technically it should be retired.
Although the Boreo is marketed by Petzl as a Caving Helmet (for which technically there is no standard; most people go with the climbing helmet standard) if I was buying helmets for predominantly group caving I'd go with the Panga. They fit a huge range of head sizes (although there's a lot of helmet poking out the back on a very small head), super easy to adjust and fit, and internal components can be replaced. You're not going to get the same level of protection if you fall and bang your head on the side as you would with the Boreo, but they'll last a lot longer.