"Absent-minded" refers to a person who is not paying attention, or to things that such a person might do from time to time. A very common phrase is an "absent-minded professor" someone who is very smart, but who is too absorbed in his or her own thoughts to notice was is going on around them in the real world. "Level-headed" means someone who doesn't get taken over by his or her emotions. And "old dear" is a way of talking about and older person who is very nice and gentle, but perhaps not totally still able to think clearly. It usually is used for a woman, but sometimes it can refer to a man too. He is a pain in the A$$ is a very common way to describe a person who has a prickly personality.

"Absent-minded" means doing something silly BECAUSE you are lost in your thoughts and not paying attention. You are listening to a podcast, you walk into the kitchen meaning to put your dirty coffee cup into the sink, and you absent-mindedly put it into the refrigerator instead. "Level-headed" means always being sensible. It means getting so excited that you do extreme or unwise things. "I was scared because the stock market dropped, and I was ready to sell all my stocks. A friend of mine said 'ALL of them? That isn't very level-headed.'" Level-headed means you are always keeping things in balance. "An old dear" means "dear" in the same way you feel about a child. You don't respect her completely, you think she is silly, but she is sweet and kind and you love her anyway. A perfectly acceptable substitute is "He's a pain in the neck." It's an idiomatic expression, it's always either the "neck" or the other place. And yes, someone whom you find annoying for any reason is "a pain in the neck." In current US culture, "pain in the butt" is probably acceptable in most situations.