(a) is technically right … but in English, the subjunctive is obsolete; and this in is just one last vestige of that tense.

If I hadn’t grown older, things would be different.

There is a subtle difference between that and a different scenario where, let’s say somebody might have said “I turned up at the party last night. I thought you were going to be there” and I replied “No, if I WAS going I would have told you”. It’s not “speculative” I am saying “this is precisely what I would have done”.

(C) If I were Younger, Things would be different.