What does 'A little long in the tooth for all this' mean?

[Olena]

English Language Tests, Intermediate level

ESL/EFL Test #290 "English Slang Idioms (12)", question 4

"I am not surprised that the Sergeant Major couldn't complete the brigade. He will probably retire soon. He is getting a little ......... in the tooth for all this."

(a) long
(b) gray
(c) short
(d) weak

English Language Tests, Intermediate level

ESL/EFL Test #290 "English Slang Idioms (12)", answer 4

"I am not surprised that the Sergeant Major couldn't complete the brigade. He will probably retire soon. He is getting a little long in the tooth for all this."

Correct answer: (a) long

Your answer was: incorrect
"I am not surprised that the Sergeant Major couldn't complete the brigade. He will probably retire soon. He is getting a little gray in the tooth for all this."
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what does that mean? "A little long in the tooth for all this"

Olena

[Mister Micawber]

.
Long in the tooth = old
.
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Native English teacher at Mister Micawber's