Should a wife tell her DH to avoid someone..?

In principle, no. But if the DH in question is being pervy or creepy or appearing to have an emotional affair - anything inappropriate, basically - the DW should be able to call him out.

No. No one owns anyone else, even in a marriage, and no one gets to police their spouse's relationships.

If you don't trust someone you're in the wrong marriage/relationship and should leave.

Any attempt to tell someone not to see someone (or even ask them) is a tacit admission that there's no trust and therefore no relationship. If you have to tell someone they've crossed a line the ship has already sailed. There is no value in being in a relationship with someone if you have to set boundaries on their behalf.

I get what you are saying. Whoever would opt to do such immature nonsense, I could never be in a relationship with. I feel most happy with a grown-up woman, rather than an insecure teenage girl. Luckily, my wife is the former...

No never. One thing I absolutely cannot abide is controlling behavior and the emotional blackmail that comes with it. Dh knows this when I met him. In a past life I would immediately end a relationship with as much as a 'are you looking at him' comment. Dh befriends who he wants, as do I. What he says or does when I'm not there is none of my business as long as he isn't unfaithful. Likewise he never comments on my friends or where I go or what I do. If he doesn't like it tough luck. It's worked for us for 30 years and we've rarely had conflict about it.

In which case the wife is right to have concerns? Would you want your DH going out of his way to be extra helpful to someone he finds attractive?

It doesn't really matter if you think she's right or wrong. Her feelings are her feelings and if something about her DH (and your?) behaviour is making her feel unhappy and insecure, of course a decent man, who cares about his marriage and is not having an EA would do whatever he can to fix that.

Did you mean to put “unhappy and threatened “ when you put “ happy and unthreatened” in your OP ?

Perhaps it happens occasionally, but why dont they seem to feel the same need to be friendly and helpful to their older ugly colleagues. It's not just showing Jane how to use the photocopier, if the DW is concerned. It's messaging out of hours, 121 coffee, late nights "working". Things these men don't do often with their good male friends.

Conversation. Not insecure demanding, that is the key. If someone has worries, dislikes, in a healthy relationship it could be discussed. But nobody has the right to flat-out tell or give ultimatums on talking to someone else.
Always imagine the situation in reverse - what if my partner did this or that? How would I feel? It needs to be talked about like grownups, not demanding and being extreme like 10-year-olds who try to tell each other who they can and can't be friends with.

It is amazing, the way so many middle aged men develop close friendships with attractive younger women, and somehow not the older fatter women they work with.

I think if the man is prioritising his marriage and the women has expressed unease, she wouldn't have to ask.

They generally don't suddenly get a new male friend and need to go out to dinner etc

No, they go to the pub or the football or for a bike ride or to play golf or Warhammer or FIFA or any other male-coded friendship activity. Men actively make friends with "Brian from Accounts" all the time.

But you are rather moving the goalposts. The OP talked about men being "chatty" with attractive younger women, not "taking them out to dinner."

Ultimately, it comes down to trust. I know I can enjoy the company of an attractive colleague of the opposite sex without being in danger of being seduced by them and I have to trust I'm not unique in this.

I have been married for 30+ years and if I voiced similar concerns to my Husband I know he would listen and take them on board as he respects me and my feelings are valid. I’m not jealous or paranoid in any way and he knows I would only raise things if they were bothering me and vice versa. It should never be a think twice moment if you need to speak to your other half about something because you should always have that 1 thing in your relationship that you can go and talk to them. I don’t understand why these days people are having communication problems. You should be able to talk to your other half about anything.

The MN trope about how "it's never Brian from IT or Janet from accounts they make friends with" is so tedious and divorced from reality, so thankyou for calling it out.

Men in every workplace I've ever been in are friends with other men, older women and younger women. Those friendships can be near-identical in terms of messaging and socialising but the territorialism and objections only happen with the last category.

The aspect that pisses me off the most isn't the "poor men" one, it's the fact that such territorialism can really harm younger women in the workplace. Normal mentoring style relationships, along the exact same lines as a senior man making friends with a younger man, can become fraught for no reason at all. One of my good friends went through absolute hell in her 20s because her male mentor's wife decided, with no evidence or basis in reality, that she was some sort of femme fatale out to steal her husband. My friend literally had to leave that company eventually.