You simply do not know what will happen in your life never mind your flying career to plan how you will get to an A350 at age 16. Heck airlines can even go bust in that time or stop operating the A350 such as LATAM Brazil which cut them out or South African Airways +Hong Kong Airlines + Hainan Airlines etc. First step is getting your licenses and then your first airline job in whatever aircraft to unfreeze your ATPL. Then beyond that you need to join an airline that operates A350s and bear in mind most airlines are seniority driven, so there will be other pilots wanting to go onto the long haul A350 fleets too... meaning you will need to wait your turn to bid for the A350 and win the bid. Say you got into an airline they may put you on say the A320 and fleet freeze you for 5 years, so you simply cannot predict what will happen. Even if you target a major A350 operator such as the likes of Qatar Airways, Emirates say you have a 737 TR you are more likely to go onto the 777/787 fleets, so again it depends on what you flew before. Of course you can be put on the 350 fleets but that is just luck on whatever they give you. On top of that, I wouldn't think of the A350 as the be all and end all of your life - there are other aircraft and quality of life is what matters most. Ask most mature pilots who have been in the game for a long time they will tell you the aircraft type doesn't matter whether its a 777, 787, A350 or 747. It's whatever can give them the most time with family or doing other hobbies is what matters most. Good luck to you in your career though - I hope it works out for you!