The maximum age for joining 4 Para is theoretically the same as the rest of the TA.
The stumbling block at 36 is not your age, but potentially your fitness. If you run the risk reducton run (2.4km) at your TA Recruit Selection weekend (TARS) in under 9.30 you stand a fair chance of getting through the training, and will definately need to be under 10 minutes to even be considered for acceptance on the cadre.
Joking aside, if you are a 10 minute or under 2.4km runner, don't wait until then to apply. The recruit training programme is demanding but progressive, and will get you to the level of fitness you need to have for P Coy, provided you apply yourself in your own time.