How frequent is the tram service from Manchester Piccadilly to Old Trafford and what are the charges?
Note on locations
Note that the Metrolink stop named "Old Trafford" is for the Cricket Ground, not the Football Ground. For MUFC, you should get off the tram at Pomona, Exchange Quay, Old Trafford or, from 2020, Wharfside and walk from there. This question should be useful to you.
You can see Tram Frequencies for leaving Picadilly on this page fromm Metrolink (also possible to get them from other stops). Most services run every 12-13 minutes, although some run every 6 minutes during the day. On Match days, extra trams will be put on before kick off and after the final whistle. As you can pick between several different tram lines (see this map, once the work in St Peters Square has finished for the lines), you will probably normally have to wait no more than 3-4 minutes for a tram.
You can use this tool provided by Metrolink to check the prices at the time you travel. Currently, the journey would cost you £3.00 for an off - peak (after 9:30) return from Picadilly to Exchange Quay. There are also options for singles, on peak tickets, and all day travel cards. There are machines that take (chip and pin) cards and cash on all platforms, and there is also a mobile app called GetMeThere that can be used to purchase tickets. It's also possible to load up an ITSO type card with tickets (Manchester's variant is branded "My GetMeThere", but for a tourist, this is not worth the bother.
According to Manchester's website:
The other quickest way of getting to Old Trafford (especially for Sunday matches) is to grab a lift on the nearest tram/metrolink. Trams travel every 12 minutes every day of the week.
And from Metrolink:
The price for an adult return ticket is £3.20 and £1.20 for a child return ticket. One day travelcards and family travelcards are also available to purchase from the ticket machines.