Personally, I'm against dropshotting the elderly. If you must, though, please use a very flexible racquet with loose stringing. I would also limit this to grass courts. They'll get a softer landing.
From another point of view: I play a very soft game against the more decrepit members during club social round-robins. Later, after changing opponents, the elderly see me playing a full-speed real serves game on the next court, and I often hear them say "well, why didn't he play that way against us?" That lets me know they were physically gone upstairs, too. It makes me glad I played the way I did against them. I should note I'll probably be in their shoes in another fifteen years. (If the shoes are the right size....).
In short, play with decency. Don't drop-shot the eldely, and don't hit the occasional underhand serve to your fitness peers. If you're physically disabled, then hit all your serves underhand. Cheap shots make tennis a cheap game.