How reliable are the biological age tests you can buy online?
The available studies offer no answer to this question; whether online biological age tests are reliable cannot be inferred from them. Look critically for independent validation studies before purchasing such a test.
The available studies do not cover consumer biological age tests sold online. The sources address research into biological ageing in a broader sense, but not a single study tests or evaluates the products you can purchase online.
That means we cannot say anything, on the basis of this information, about how reliable such tests are for individual use. Whether a test measures what it claims to measure, how consistent the result is when repeated, and whether the score predicts anything meaningful about your health: the available sources provide no answer to any of these questions.
If you are considering buying such a test, be aware that the scientific backing of consumer products is far from always publicly available or independently verified. If in doubt, discuss the result with your GP, who is better placed to assess the context.