It's a little more complicated than just living together. The couple has to have presented themselves as married and has publicly announced the intent to marry
Here are the facts about common law marriage in Kansas. note the age requirements.
“The state of Kansas shall not recognized a common-law marriage contract if either party to the marriage contract is under 18 years of age.”
Additionally, the state will only recognize a Kansas law on common marriage in if the two parties meet the following three elements:
1. There was intent and agreement in praesenti to be married by both parties
2. There has been continuous cohabitation between the parties
3. There is a public declaration or holding that the parties are husband and wife"