If you have the choice, definitely don't leave them to 2 years old before slaughter. All of the older pigs (Tamworth and Saddleback) we had to have slaughtered were very very fatty, the meat was dark and tough and had no resemblance to pork at all, in fact it was disgusting as anything other than a sausage. The fat is very unpleasant, loose and wobbly too, not like the nice firm fat on a young overfattened pig. I would say up to 9-10 months old is the maximum I would leave them before slaughter if I wanted to get a decent pork product out at the end.