My eldest two DSs were 5 and 4, then 7 and 6 when DSs3 and 4 came along (my goodness, that reads really confusing: I have 4 boys, now aged almost 8, 7, 3 and 10/12).
The only 'rules' we ever had WRT to the latest baby was No Carrying Around. I always allowed them to hold their new brother: sitting right back on a sofa, their receiving arm support by a cushion and a responsible adult hovering nearby. I have never worried leaving them together in a room with the baby on a playmat and me just nipping out.
The 2 older ones are brilliant with new babies, v protective and gentle. DS3 who was 2 when No4 arrived however... but that's toddlers for you - they are all psychopaths a law upon themselves 
!
I also feel it is a good thing to get them involved in looking after the baby without making them feel it is their 'job' (my DS1 takes everything awfully seriously and I have to watch that he does not assume parental responibilities for his younger brothers).
BTW, the now sturdy, crawling 10 month old loves being held (and carried
) by his eldest brother. And tickled - he gives bellylaughs and giggles of a unique intensitiy when DS1 plays with him
Enjoy them together! 