As already said 'seniority is everything' in BA, their is a LOT of luck how 'the list' looks when you join and how many older people lie ahead of you and will retire before you do.
I joined early 30's as a DEP and will retire at age 55 with a seniority of around 1150, maybe just enough to be a very junior long haul Capt. in my last year or so.
Other airlines I have worked for seniority only mattered for time to command, BUT in BA becuase of the 'bidline' system for rosters it makes a huge difference to your lifestyle, where you fly to and what days off you get (being junior expect to work every weekend).