
The most common sign of dementia in older cats is the cat starting to miaow a lot, sometimes during the night, sometimes just wandering around the house miaowing/crying loudly. You don't usually see temperament changes. Rescued mogs unfortunately CAN be very bad tempered without having anything wrong with them.(After all, they can have had all sorts of bad experiences, and won't have been bred for temperament.) One of mine bit my mum so badly she ended up in hospital for 8 days and had to have surgery on her arm -he was ten then and had always been grumpy but one day just had enough and she pushed him too far.
However laying in the hallway and jumping at passing legs, dogs etc that IS just play -but no doubt the play of a cat that wasn't ever taught manners by his mother/siblings (i.e. he doesn't know when he's gone too far), so no doubt removed far too early from the mother. (Almost all moggy kittens are sold/rehomed at 6-8 weeks, but in reality no kitten should be sold earlier than 13 weeks and being taken away too early does often mean temperament problems for life. Try using a waterpistol and spray him when he does it.