Capablanca likely could have if he hadn't died relatively young (age 53). He had been staging a serious middle-age return to high-level play at the time and was an extraordinarily gifted player: when Arpad Elo developed the rating system, he compiled ratings for past players and ended up assigning Capablanca what was at the time the highest rating of any player in history.

Meanwhile Garry Kasparov is now the same age Capablanca was at his death, and continues to play at a very high level; he still holds a FIDE rating of 2812, down from his peak of 2851 and historically second only to Magnus Carlsen's peak of 2882, and in April finished third in a blitz tournament where he put up a 2-1-3 record against eventual champion and GM Hikaru Nakamura.

Reshevsky, many times over.

"In 1981, at the age of 70, he tied for 3rd place in the U.S. Championship. In 1984, at the age of 72, he took first place in the powerful Reykjavik Open, which featured many grandmasters"

http://www.chessgames.com/player/samuel_reshevsky.html

Sure. Keep in mind that the top GMs have ratings of 2700 and above, while some GMs have ratings as low as the 2400s. So a top-level GM still has a large amount of age-related distance to fall before he won't be a match for a "low-level GM".

Vasily Smyslov was a world championship candidate at age 62, and lost to Kasparov in a match.