My DS is almost 9 months old, and I can see his pulsating all the time. (It helps that we gave him a haircut this week - it's a lot more noticeable with his hair so short.) That said, when he was born, the midwives told me he had a very large soft spot, so who knows.
However, my not very scientific investigation on the intenet seemed to turn up info that the average age for the soft spot on the top of the head to close is between 12-14 months, with 9 months considered early and 24 months on the later end. I agree with the pps. If you think it is already closed at 5 months, I'd definitely call your ped.