Ill age him up to 28. I didnt think a 2 year age gap would pose much of a problem; its hardly uncommon.
As for a timeline...
18 - Graduate in Hong Kong, goes to Cambridge
21ish - Romance
21.5 - Graduated Cambridge 1 semester in advance, recalled to Hokoan
23 - Finished Mandatory Naval Service, Goes into Intelligence
5 years experience between field work and administrative. I would assume a 2 year field/3 year desk split.
28 - Now
So yes, 5 years experience is a VERY rapid promotion and would be unrealistic in any western or major power. However I am assuming that 1) He is an exceptional field operative and a capable logistician from a desk and that 2) being the grandson of the most powerful Admiral in the country speeds things up a bit. Cambridge probably doesnt hurt.