Those apps recorded $195M in 2019 – increasing by over 50% compared to previous year
Apparently, such evolution has been originating from millennials’ lifestyles, such persons chosing to marry later in life and delay having children; and because of that, it seems they allow themselves more time to remain self-focused, compared with their parents’ generation.

Also, internet acces had a more positive advantage for millenials when comparing to the previous generations: they have thus an easier access to information and therefore become better informed about welness and self-care.
Calm and Headspace (two mobile apps) registered (final figures not available though) $92 millions in 2019, and $56 millions respectivelly. Also, they have 24 millions new users in 2019 (Calm) and 13 millions (Headspace).
Source: https://techcrunch.com/2020/01/30/top-10-meditation-apps-pulled-in-195m-in-2019-up-52-from-2018/
Next years I think millenials will determine more and more the market evolutions based on their interests, toward a direction or another. Will be interesting to watch these trends!