"In retrospect, I wish I had been stricter with him from an earlier age!" is the only correct sentence. It clearly tells me you weren't strict enough. The other sentence is simply confusing, as the "would have been" part is pure speculation. You're wishing for a wish.

Had been and would have been both sound right to me. I am not completely sure though if there is some grammatical technicality that make them wrong. Other than that, outside UK, the more common comparative form of strict is "more strict" rather than "stricter". Also, "earlier age" sounds a little weird to me. I don't know if that is because it is incorrect or because "early age" is used much more. I have not heard many instances of earlier age before.