pocketsquare guy is right
joining late, but this is a problem i have had to skirt around too. it's almost entirely about fit and cut. old men wear loose comfortable clothing, so that appearance is verboten. beware those awful in-house department store jackets and shoes (Marks & Spencer etc) - the cut is all wrong, impossible to look good in that stuff. avoid jackets which are too long, low button stance, or with closed quarters - classic old man. avoid shoes with short vamps - classic old man. avoid high waisted pants - classic old man (you want lower rise, with slim leg, to keep the crotch fairly high, and lengthen the leg).
i think the style is far less important - outmoded fashions like wider lapels or lower gorge can still look great if the fit and cut is sleek. pagoda shoulderline is a bit hard to work with though.
combinations is the other thing - what goes with what. if in doubt, don't overdo the number of potentially old man elements you have in one outfit: brogues, slacks, sports jacket could put you in the old zone, but substituting slim jeans will help, or wearing the elements in a different way. some will hate me saying it, but mixing it up / adding edge is a differentiator. also you need to be confident in your own style, find an angle, and not look like you raided grandpa's closet.
when you look like your clothes / style is what you wear all the time, then it will be right. if you look like you dressed up, then it's all wrong.