Antonyms of adj youthful

1 sense of youthful

Sense 1
youthful, vernal, young -- (suggestive of youth; vigorous and fresh; "he is young for his age")

INDIRECT (VIA young) -> old -- ((used especially of persons) having lived for a relatively long time or attained a specific age; "his mother is very old"; "a ripe old age"; "how old are you?")

Similarity of adj youthful

1 sense of youthful

Sense 1
youthful, vernal, young -- (suggestive of youth; vigorous and fresh; "he is young for his age")
       => young (vs. old), immature -- ((used of living things especially persons) in an early period of life or development or growth; "young people")