I think there is a strong combination of factors at play:
People are coming into the workforce much later in their lives than before. Gone are the days you could leave school at 16. For a large proportion of the population their first full time proper job won't be until they're 21/22 when they come out of university, which they had to go to in order to try to get a decent job (NB I don't necessarily agree with this, but it is the prevailing message to school students).
And don't forget the tens of thousands of uni fee debt; something that previous generations did not have to deal with to such an extent, if at all. £100 less in your paycheck every month is not to be ignored.
Jobs are harder to come by and competition is fierce, even if they're not very good jobs. So messing up at work is likely to be more worrying for people as they won't want to lose their job because it may be difficult to find another one. Job stability is poor.
This is especially pertinent when you consider how costs of living have risen much faster than wages. The fact of the matter is that young adults now have much less disposable income than those of 10 or 20 years ago. They have less savings and safety nets are small to non-existent for many. This adds to stress levels.
Housing is also very very expensive compared to the past 20 years. Rent is through the roof and is often transient. Many young adults cannot afford to buy a house because they cannot save a deposit due to paying high rents, so they have to rent because they can't buy... And so on and so forth in a vicious circle.
Interest rates are also very very low. It is even harder to save now.
Widespread exposure to technology (social media etc) is also a known stress factor. And in part this is entirely voluntary, but we shouldn't ignore how addictive technology can be.
Maybe adults are less resilient now than they were (it's hard to tell, as the plural of anecdote is not data, as we all know), but for my money I'd say humans are just as resilient as they've always been, but they have a heavier combination of economic and societal factors weighing on them than recent generations had.