Yes, except that nobody can directly observe the past to know the age of the universe, it is all circumstantial, and radioisotope dating is not strictly physics (it was covered in chemistry); and once you apply it to rock ages because you must presume initial conditions to know the age, and once again nobody was around taking measurements at the beginning to know what the original composition of the rocks were.
Nobody can directly observe an electron, either. The circumstantial evidence of it's existance is sufficient for me.
Regarding radioisotope dating, it IS physics rather than chemistry. Where you have an isotope with a known half life, and can measure the proportion of the isotope and its decay products, you can have a pretty good idea of how long that isotope has been sitting there.