The syndrome has been reported in people aged from 18 to 79. Of those, 51 were women and 28 were men. There is some evidence the risk of the side effect decreases as people get older.
However, the main reason for drawing a line at age 30 and below relates not to a person's increased risk of the side-effect - it is after all exceedingly rare - it's to do with the balance of risk and benefit of having the vaccine. Regulators have concluded that above the age of 30, the risk of serious harm or death from Covid is higher than the very small risk of a potentially fatal blood clot.
Below the age of 30, the risk of dying from Covid, if infected, is less than one in 2,500. And of course, the risk is lower still because not everyone gets infected.