What causes a behavior change as they get older

As my staffy has gotten older is seems his tolerance to other dogs has gone. He was a rescue dog and i have never had any issues with him til now. He doesnt seem to like small dogs (like jack Russells ) he is ok with same size or bigger dogs. He also only seems to be aggressive when he is with my brothers ddog (amstaff x )does he behavior affect him? Any help would be great.