My rennet has been in the fridge for a year or more. Can I still use it?

Probably not. Rennet doesn’t  go bad  so much as it loses its ability to coagulate milk after about 1 year. You can still try it, but you will need to use more and be willing to possibly sacrifice your batch. It it’s been more than 1 year, you will want to get some fresh rennet.

http://shop.homecheesemaking.com.au/vegetarian-rennet-50g-enough-to-set-250l-of-milk/

Rennet

An extract from the membranes of calves’ stomachs which contains rennin, an enzyme that aids in coagulating milk or separating curds from whey. Rennet-like enzymes, also used commercially, are produced by selected fungi and bacteria and are referred to as “coagulant” instead of rennet.