His good character has vanished, and his bad character has taken its place. أَدْبَرَ غَرِيرُهُ وَأَقْبَلَ هَرِيرهُ.

Good character refers to his pleasant disposition, while bad character refers to his unpleasant disposition. This proverb means that what was pleasing and admirable about him has disappeared, replaced by what is disliked, such as bad character and other negative traits.
This proverb is used to describe an old man whose character has deteriorated.
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المثل العربي «أَدْبَرَ غَرِيرُهُ وَأَقْبَلَ هَرِيرُهُ» يقال لمن ذهب حُسن خُلُقه وما كان يُعجب الناس منه، وظهر منه سوء الطبع والضيق، وخاصةً إذا تقدّم به السن.
ولا يوجد مثل إنجليزي مطابق لفظًا، لكن من أقرب الأمثال والتعابير في المعنى: There's no fool like an old fool.
«لا أحمق كالأحمق العجوز.»
يضرب لمن يزداد سوء تصرفه أو حماقته مع الكِبَر.
Age does not always bring wisdom.
«ليس التقدم في السن دائمًا جالبًا للحكمة.»
وهو أقرب إلى المعنى العام للمثل.
He has grown old and bitter.
«لقد شاخ وأصبح مرّ الطبع.»
تعبير شائع يصف من تغيّر خلقه إلى الأسوأ مع الشيخوخة.
Cranky with age.
«أصبح متجهمًا أو سيئ المزاج مع التقدم في السن.»
وأقرب مقابل من حيث الدلالة على فساد الخلق مع الكِبَر هو:
Age does not always bring wisdom.
«ليس التقدم في السن دائمًا جالبًا للحكمة.»
وإذا كان التركيز على التحول من اللطف إلى سوء الخلق، فيمكن أيضًا القول:
He has grown old and bitter.
«شاخ وصار سيئ الطبع.»
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الغَرير: الخُلُق الحسن، والهرير: الكراهية، أي ذهب منه ما كان يَغُرُّ ويعجب، وجاء ما يكره منه من سوء الخلق وغير ذلك.
يضرب للشيخ إذا ساء خُلُقه.
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The Arabic proverb “His youth has turned away, and his snarl has come” is said of someone whose good character and the qualities that once impressed people have faded, and who now displays ill-temper and irritability, especially as they get older.
There is no exact English proverb, but some of the closest in meaning are:
“There’s no fool like an old fool.”
“There is no fool like an old fool.”
This is said of someone whose bad behavior or foolishness increases with age.
“Age does not always bring wisdom.”
“Growing old does not always bring wisdom.”
This is closer to the general meaning of the proverb.
“He has grown old and bitter.”
“He has grown old and become bitter.”
This is a common expression describing someone whose character has deteriorated with age.
“Cranky with age.”
“He has become sullen or ill-tempered with age.” The closest equivalent, in terms of its implication of moral decay with old age, is:
Age does not always bring wisdom.
“Growing old does not always bring wisdom.”
And if the focus is on the shift from kindness to bad character, one could also say:
He has grown old and bitter.
“He has grown old and ill-tempered.”