It's not being 40 that makes it acceptable ... it's become mainstream for all ages. [I may be biased, but I think us gay guys really wear the look well.]
I think it makes all of us more "age indeterminate."
<insert story here ... like I need an invitation>
A couple of years ago I was working with a young gal (21) on the evening/night shift. It was her first "late" night (at the time it was not yet 10 p.m.) and she was lamenting how she had never worked that late before and (looking at me, with my gray hair) was trying to avoid saying that it must be really tough on us old people. Later that evening age came up and I asked her how old she thought I was (I was 54 at the time). Not wanting to hurt my feelings, she guessed I was in my early 80s! It's provided plenty of laughs over the years as I've retold it, and I merely translated her assessment into "old."
Last week I was working with someone else, but also early 20s - she's only known me a few months, always sly. Something came up about age and I said I could easily be her dad's age (or older). During further discussion she'd guessed I was early 40s.
I don't take either situation to mean anything significant, and age doesn't bother me a bit. My years have served me well and I've lived and learned a lot -- and have a lot of living and learning yet ahead.
But I think Sly removes one of the primary "age barometers" which people rely on. Now they've got to go much more by either my silly antics or my extensive knowledge of history (like things in the 1960s and 1970s).