Think we started around 2 months - but Katie has always had very strong head control. Initially we only did it for 5 mins at a time - she loved it but got tired quickly. Now she's nearly 4 months and she's OK in there for an hour or so and absolutely loves it! It's Katie's favourite pastime - we go to the local shops, to church, everywhere...
I would give it a go - it's the only way to find out - but just for a short walk around the block initially, so that she doesn't get too tired and so that you can easily turn her around if she doesn't like it.
Have fun!
Bryony