one who becomes mature at a later age than normal

ُأعد من الأشخاص حديثي العهد عندما يتعلق الموضوع بكذا وكذا

In this context I'd say sth like

لقد دخلت عالم الموضة والاعتناء بالمظهر متأخراً بعض الشيء
لقد تأخرت في دخول عالم الموضة

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Note added at 23 mins (2009-01-08 12:44:25 GMT)
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(translation above doesn't include "when it comes to stuff like.." but then I was only suggesting a translation for "late bloomer")

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Note added at 1 day57 mins (2009-01-09 13:17:56 GMT) Post-grading
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Note post-closing: A late bloomer is "someone who discovers his or her strengths later than expected." (www.byamycohen.com/Pages/between.html ).
So the expression refers to someone who has ALREADY discovered their strenghts, even if late. To translate it as بطيء الفهم in your context would be, in my opinion, misleading, as that doesn't convey the idea that that person has discovered any kind of strength, but on the contrary implies that the person is still slow whenever it comes to fashion and grooming.

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Note added at 1 day1 hr (2009-01-09 13:40:43 GMT) Post-grading
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Hi terry777 In reply to your note: Precisely: This person has released his own cologne, and is promoting it. He is telling us why it took him so long (he was a late bloomer), but NOW he is a kind of "expert" in fashion and grooming (which is why he has his own cologne). If he was بطيءالفهم (i.e slow-witted) when it comes to fashion, he would still be unqualified to release his own cologne. People would not want to buy cologne from that sort of person.