The problem is if you stop her (or tell her off), it can escalate the situation. This is because she'll stop doing it (because she knows she shouldn't) but she may just snap one day because she is just a dog so instincts take over. What you need to do is avoid putting them in situations where one dog feels the need to 'tell off' the other.
It needs quite carefully handling and completely depends on what the triggers are and learning them. What you don't want to do is inadvertently worsen the situation i.e. treat them both as equals because, tbh, they're not. You have an older dog and a younger dog and they need to be treated with that in mind. Your older dog is unhappy (which is why she's telling the younger one off) and if, until now, the younger dog has accepted that then that's good. It's much better than two dogs duking it out all the time.
In terms of triggers, if for example your younger dog is being fussed lots and 'acting out' to get attention and you're reacting to that or giving her lots of fusses or she's being loud and irritating that can set off jealously.
If you're saying something like 'Penny, no' (for example) that can sometimes trigger the older dog because they think the other dog has done something naughty (my cocker reacts like this if I tell my other dogs off in a particular voice).
One of my cockers triggers is actually the other dogs going through any door before her. Which sounds completely nuts. But it is. She likes to go through straight after me, with them behind her. She tells them off if they go through before her. No idea why. But it's her 'thing'
I don't think there's a typical age for it but, of course, as dogs grow up and become more confident in themselves and their surroundings they become more likely to push each other.
To help manage my cocker, I did the following, which really helped (and is something I've kept doing with successive dogs)
- Feeding the oldest dog first, then the middle and then the youngest
- No toys left out for them to fuss with outside key play times
- No long-lasting treats or raw-hide bones
- Lots of exercise and mental stimulation (2-3 hours of walks a day as mine are energetic breeds)
- Lots of individual 1-2-1 time with me.
- A 'safe' space for each of them and time away from each other during the day.
- No overstimulating any of them including rough games of tug of war etc. (particularly my current puppy as he's mouthy and I realised my cocker doesn't like seeing him mouth me)
- If I think they're getting a bit too excited I walk away from whichever one it is
Don't forget that dogs will give off signals - as yours are doing. So your eldest, by telling off the younger one for whatever she is doing, is saying 'stop it.' You need to respect that and find a balance between not endorsing the behaviour but not immediately rushing to the younger dog and saying 'there there, it's okay.'
Behaviourists may have other suggestions, but this is a long-term dog-dynamic issue that will need management rather than a behavioural issue (such as SA) to be solved...
I hope that helps! I'm definitely not an expert, but the above is how I approached/managed my cocker and is has worked. It is tiring, but it's made her a lot happier