Medics and police were called to the Cyclopark complex in Gravesend this afternoon.
Two air ambulances – Kent's and another from London – were drafted in to help with what police described as a "medical incident".
It happened at about 12.20pm at the park, where there are facilities for cycling, BMXing and skating.
A police spokesman said they were called to the venue, off Watling Street, at 12.21pm.
He added: "Officers attended the scene to provide assistance to crews from the South East Coast Ambulance Service."
A Cyclopark spokesman said: "Our staff have been assisting the relevant authorities appropriately with their investigation and also supporting our staff members who helped to deal with this tragic incident.”