This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. Informal An elderly person.

n. Someone who is old-aged

n. An old person.

n. An old or oldish person; a man past middle life.

n. In the British navy, a midshipman of four years' standing, or a master's mate.

n. an elderly person

"He replied, “Yes, and the oldster is become my good friend: my way is his way and he would well I had need of him.”"

youngster   

grownup    adult   

sexagenarian    dodderer    back-number    octogenarian    centenarian    fogey    dodo    old boy    fogy    greybeard   

bannerman    aprosodia    fourth-year    fighting-man    garden-bed    sheepdog    cavalla    artizan    carlin    quodammodo