DEAR MISS MANNERS: I’m 75 and in reasonably good shape, physically and mentally. Recently in my water aerobics class, someone who is one month younger asked me, in a crowded area, “How does it feel to be the oldest one in the group? Ha ha.” Then my brother-in-law, who is six months younger, said at a family gathering, “How does it feel to be the oldest one here? Ha ha.” I would love to have a snappy/sassy response ready if this happens again. Any suggestions? GENTLE READER: “It feels good to have outgrown the need to answer silly questions. I guess that’s what you have to look forward to. Ha h…