Seniority lists are, at least in the UK, are open to a challenge in the courts given the new age discrimination laws i.e. you cannot sack your youngest employees when you are downsizing
Technically, seniority is nothing to do with age. Last in first out is a well known and accepted way of dealing with forced redundancy.
A 27 year old pilot could have 6 years seniority and will not be sacked at the expense of a 50 year old pilot who only joined 5 years ago.
Two completely different things.