Nice pictures of Bob! And a happy belated birthday to the girls.
We do "guesstimates" on birthdays for our girls, too. Finn was a stray, so small that I thought she couldn't possibly be more than 4 months old when she adopted us. The vet, however, said she had to be at least 8 months old due to the adult teeth she had. Bex is more easily pinned down. We have her age within a couple of weeks, since she was between 5-7 weeks when we found her at the shelter. They are about 3 years apart in age.
My next mission, when they are both much older, is to adopt two sisters, both tiny. I'd love to watch the dynamic of litter mates, as opposed to the mixed bag I've always had with cats.

They never cuddle, only form occasional truces (share the bed with us, on opposite ends), but the rest of the time it's spy vs. spy - complete with ambushes and treat-stealing.