Tough one this. Half of me says "get out, I've booked this seat. If you want a reserved seat book one yourself"; no reason why being old is an excuse. One the other hand, however, could you, really, ask an OAP to stand. Don't think I could.
In a similar vain, though, my wife and I were once on the Highland Chieftan in the days when smoking was allowed. We were in first, no-smoking as the wife doesn't smoke, and I walked in to the rear coach, sat down and proceeded to light up. The lady opposite asked if I could refrain from smoking because her son, sitting next to her, was asthmatic. No love was lost here and I politely reminded her she was sitting in the smoking section and that she should move, not me. I got the daggers from all around but in this instance felt I was in the right.
She wasn't that bothered because she didn't move. If my kids suffered similarly I would have not sat there in the first place.