Hawaiian shirts have their season - high summer in hot climates, and that's IT. No wearing 'em past Labor Day (or Columbus Day, if one lives in Arizona, because it stays hot until about that time).
Gear T-shirts can be cool if worn underneath an unbuttoned and un-tucked dress shirt. Same for sports T-shirts. I do have a weakness for hockey jerseys during cold weather months, but that is because I have so many of them from my playing days. I am very careful about which ones I gig in, however, and am guilty of wearing gear T-shirts under them (or the Bass Place T-shirt, to let everyone know about my town's best spot for all things low-end). Gear T-shirts are great for load-in/out, however, as they often provide additional visual cues to otherwise unmindful folk that you are trying to accomplish something, and they need to get out of your way.
Sunburst finishes are timeless, and we all know about micro-amps and more efficient, lighter-weight cabs being the new standard.
Boutique basses definitely make you look old, but can also make you look successful; there is a delicate balance there. It really depends on what kind of boutique bass, what wood or finish it has, and how many strings.
Mullets are probably not a good idea, but then again, they actually work in some scenes.
Music stands probably do make you look old in most situations, save for jazz combos. I only use them for our rock karaoke gigs, since we have over 300 songs to choose from, and some we simply do not play that often. Since we don't have the video monitor, and the guest vocalist cannot be relied on to perform well, we have to be ready to jump in over them. I am working on a volume pedal mod that is XLR in and out, to be able to mute or cut off the guest vocalist on the fly.
Old is really more about attitude than appearance, although beyond a certain point in time, it simply is what it is. I'm 45, and a good day is when I pass for under 40, or when a late-20's gal shows interest (and they still do). Of course, I also know how to be non-threatening, so the really fine young hotties are comfy with my photography for those Chicks at Shows threads. Some even pose with my gear for me; they get it, and I do get the pleasant surprise of kisses and phone numbers/e-mails in my pocket (or even dropped at my feet while still playing). The cougars adore me still.
Of course, you don't have to be old to enjoy an energy drink, but I do keep a shot of 5-Hour energy in my accessories bag, just in case, and usually brew myself a custom espresso drink at home before leaving for the gig. It also helps to bank sleep when you can.
I'll keep trying to defy Father Time as long and subtly as I can. When it starts getting obvious, I simply won't fight it any more, and hopefully, that's another decade away at least. Even then, biker and jazz gigs will still be happening.
If you sound good, have stage presence, and entertain well, it really does not matter how old you are (at least in a cover band context); people will respond to your vibe. Of course, it does not hurt if you are able to pull a Lady Gaga tune out of your @$$ on short notice, or honor a song request instantly. When it comes to making original music, age 30 is rock & roll death. The last band of significantly over-30 guys to make it big in rock was the Police. Other genres can be kinder, as their audiences may skew older.
Just know who and what you are, and project the aforementioned good vibe. As long as your gear gives you satisfaction, there's a strong probability that the audience will dig what you do, and that's what it is all about.
...now where did I put that bottle of Centrum Silver?