In my experience years on the job are what determine the level someone gets hired at, not skill in a specific language.

Being a senior engineer is not normally about raw coding skill -- it's about knowing how to design a complex system, knowing what can go wrong, being able to collaborate effectively with others, and so on. The specific language used is not normally going to make a big difference in those areas, although candidates who need to pick up a new language are going to take longer to ramp up.