addressing the OP:
while I may not have mentioned it, I'd have definitely thought it -dressing like you are 16 when you are middle aged is a definite faux pas IMHO. Mind you, not any law against it, and I'm certainly far from fashionable, so who am I to talk? (I wear crocs* a lot as they are comfortable, light and practical -and I really don't care if they are unfashionable).
As I've got older (in my fifties) I've definitely tried to eschew jeans and t-shirts at work -my usual thing is a collared plain shirt and chinos sort of thing. When I do hang around maybe jeans and a t-shirt then. I'm also a fleece person as well. Whenever and where ever possible I'll wear shorts as well, but again, not at work.
*mind you, I absolutely abhor flip flops, both in terms of their looks and their practicality (I also think it may be something to with the fact many trust fund babies over here will wear them at their wedding to show how down to earth and hip they are, and how much they love the beach). And don't get me started on loafers or slip ons -particularly the ones with tassles <shudder>. The most poncey looking things I've ever seen. IMHO an abomination that should have long ago been banned under the Geneva convention for offensive footwear.