It's all about what they bring to the table. Some people are clearly senior level after 2 years: they communicate well, they can mentor other engineers, they can criticize an existing design or plan the implementation of a new feature, have good organizational skills etc.

After 2 years I wasn't able to provide constructive feedback without being a little too brash, I couldn't have designed anything distributed, I wouldn't have been able to break up a gigantic feature into smaller problems etc. I've worked with people who were technically competent and mature 2 years after school.