Jacuzzi bath for baby

I live in Germany in an area famous for it's springs. We have multiple thermal spas and the rules of admission are different in each. When my children were young, I checked with some of them why this is the case:

I was told that each spring has a specific content of mineral salts and hence a different effect on the human body. For young children, the purifying / healing properties of some of the waters, especially combined with the rather high water temperature in a jacuzzi, can be very stressful for the circulation and affect blood pressure negatively. As these effects are hard to quantify and depend a lot on the physical condition of the user, each spa here calculates admission depending on previous experiences over decades (or centuries) of use. A certain "safety value" is most certainly included and usually the worst that should happen to a healthy child or adult is dizzyness and general fatigue for a few hours afterwards. Other rules apply in case of a heart condition or other sicknesses and it is recommended to check with a physician before going.

We have spas in the area that will admit babies (but recommend limiting the bathing time), others give a minimum age of up to eight years.