"So what other laws treat two adults differently? Can I get reduced sentences or fines for being a senior citizen?"
I'm not sure what you are getting at with this. The constitution doesn't set an age does it? So what does age have to do with anything.
To answer your question with one of many examples, if you were mentally unfit you are treated different come sentence time.
Right. So, except for minors and those declared incompetent to act as adults, all adults are treated equally. In fact, the Constitution explicitly requires this of our government, and the XIVth requires all states to treat all adults equally under thelaw as well, right?