The average lifespan for most species of stick insects is 1 year. Older stick insects require more water than younger ones and so the leaves need to be misted more thoroughly with water. If very old stick insects are short of water, they can start to discard legs, as you have observed. So you need to give them more water.
Other reasons for missing limbs are:
Overcrowding - the stick insects bite or knock off legs of other stick insects in their cage. Your stick insects should have plenty of room in their cage and this is especially important when they're moulting.
Fungal infection - if you suspect your stick insects have a fungal infection, thoroughly clean their cage and, if necessary, quarantine infected individuals.
Rough handling - stick insects are fragile and, like all animals, should be handled with care and respect.
Stick insects undergoing incomplete metamorphosis. It is possible for a young stick insect to regrow a lost limb but only when they moult again. This means that, if your stick insect is an adult, then it won't be able to regrow the lost limb.
