How do I get from Manchester Airport to Old Trafford?

The tram takes a long way around - it takes a bit of a tour through Manchester suburbs, and it's maximum speed is 50 mph (80km/h).

If there was a direct service, then the extra convenience of not needing to change would probably make it a superior option. But there isn't a direct service - you need to change at Trafford Bar or Cornbrook. I would suggest changing tram at Cornbrook because you can walk from one line to the other directly, rather than having to climb a bridge over the tracks.

The trains from Manchester Airport to Deansgate station are much faster than the trams into Manchester, and you can cross from the train to tram at Deansgate and pick up a tram to Old Trafford.

Since you're having to change anyway, the tram/train route via Deansgate is probably faster than tram all the way. Note that some trains don't stop at Deansgate - you would need to change at Piccadilly instead on those routes, and that's a long and awkward change inside the station.

If I were making that journey, I would check the departure boards for when the next train leaves, and whether it goes to Deansgate or not. If there is a Deansgate train in the next 10-15 minutes, I'd take it; if not, then I'd take a tram (which are every 12 minutes).

Note that the tram and train platforms are next to each other at Manchester Airport, but the ticketing systems are different and the train ticket office is at the upper level (there are tram ticket machines on the platforms), so you might need to go up to the ticket office level and then back down again, so check before you buy a ticket and go down to the platform.

@pnuts comment answers this question, I guess: The Metrolink details are reliable. The service has only been running for two weeks and presumably other sites have yet to catch up with developments.