Do you have the option of cat-proofing your garden or installing a cat run? that will let them come and go, but in a safe environment. Also, they do get used to being indoors at night, it is just that at present they are used to being able to go out. It's simply a matter of changing their pattern of living. It will be hell for you for the first few nights, but they soon get the hang of it and are more than happy to snuggle down under the duvet with you, or hijack a heater or two. Some never accept this, but they are a rare cat in my experience (I have had cats in all kinds of conditions over the last almost 40 years). So give it a go, and find some quiet toys for them to play with (catnip is a useful sedative if they like it). And slow the evening down with them. Feed them, and sit back down and give them a good cuddle when they try to go out. Being out all night is for grown-ups, not kitties, so perhaps about 18 months would be a good age as they have their full growth (more or less) and are a bit more confident in the way they carry themselves, so less vulnerable to neighbourhood bullies.
