Eldest children in any year will be born in March ..... unless they were deferred. In which case their birthday could be in January or February (or even December or more rarely November or even more rarely in October), so they would be the oldest.
Ds' birthday is in September. Technically he was right in the middle of the year age-wise, but in practice, because of deferrals, he was one if the younger ones in his year.
He has one friend (deferred) in his class who turned 18 in the January of S6 and another friend (non-deferred) who was in the same class who turned 18 in the February of his 1st year at Uni 
(Ds turned 18 on the 1st day of lectures at Uni - 17 for all of Freshers' Week 
)
He also had one friend who was only a month younger than him (birthday mid October) but was in the year below But he had arrived from abroad at secondary stage with English as an Additional language.
You say your dc is in Y11 and will have completed GCSEs this time next year, so I presume your dc will be going into Y11 (which is the equivalent of S4, which is when Nat 5s are also sat).
With a January birthday, they'd have the choice to go into S4 or S3 and have one or two years to prepare for the Nat 5s. It depends on the school and/or Local Authority whether the Nat 5s are done over 1 or 2 years and how many they normally do (usually between 6 and 8).
Highers are usually sat in S5 but can also be sat in S6, alongside or instead of Advanced Highers. 5 in a single sitting is what Unis like to see - but as far as I know, they will take circumstances like changing education systems into account.
Plenty of teachers in the Scotsnet topic who can advise 