I have another thread about my poor boy cat's health at the moment however in researching his current issues I've found out that new research has shown that you should wait until 6 months to neuter tom cats.
Vets will often do from 4 months now and that's what we did with ours; we had a brother and sister together so didn't want any 'accidents'.
However boy cat is now in a life or death situation over a blocked urethra. This can be very, very dangerous for cats and can lead to death within 24-48 hours.
The research I found says that incidences have risen in recent years since younger neutering. It's thought that Tom cat's urethras haven't finished growing and neutering them at that age causes it to stop growing any further. This leaves them with a much narrower urethra and higher chance of blockage.
(It doesn't have the same impact on female cats who have wider urethras and who don't tend to suffer from blockages).
I wish I'd known as we'd have had his sister done at 4mths and waited another 2mths for him.
We now might lose our beautiful, soppy boy so I just wanted to warn others 