If the flight was to or from an EU/EEA country, the relevant time limit for that country applies. (In the UK it's 6 years but some countries have a far shorter time limit).
Outside of the EU/EEA, while the Montreal Convention gives certain rights in respect of flight problems, they don't provide an automatic right to compensation for delays (other than for
knock-on effects, such as the reimbursement of hotel expenses for overnight accommodation). Further, there's a 2-year time limit on claims. Beyond the provisions of the Convention, each airline (operating outside of the EU/EEA) is free to determine the terms of the contracts that they offer to their customers.
Any claim for a delay with US airways would now have to be made against their parent company, American Airlines.
Information about making a claim for a flight to/from an EU/EEA airport can be found on the CAA website:
https://www.caa.co.uk/Passengers/Resolving-travel-problems/Delays-cancellations/Delays/Flight-Delays/