A little nuance...
As I approached 38/16 (as a ground branch Sqn Ldr), the DeskO told me that my options were:
1. Waive option and continue to 55.
2. Take option and go on 38th birthday.
3. Defer the option by a year. Can do this up to 3 times before making decisions 1 or 2 above. On each occasion there'll be a DeskO chat at 'deferred option date minus 6 months', at which point we go through the whole the decision loop again.
Worked very well for me... I deferred the option twice then, with 9 or so months to go to my 40th birthday, I resolved a medical issue with the CAA, pulled the handle and left on my 40th.
And now... resting prior to an 0245 report for some night freighting... I never looked forward to going to the office this much!