Many couples of all ages decide to live together -- either instead of getting married or perhaps as an interim step before determining whether marriage is right for them. If you and your significant other have decided to share a home, you are likely going to be buying things together, sharing household expenses and interweaving your financial lives much as married people do. You may even decide to buy a home together or jointly adopt a pet.
If the relationship ends, you have a lot of things to untangle. This can be made easier by drawing up a legal agreement. These agreements go by different names, but they're often called "cohabitation agreements."
Whatever you call yours, it should help define how the assets you accumulate together or even purchase or acquire separately while you're together will be distributed. If you intermingle assets -- for example, by opening a joint checking account -- you can designate how they will be divided. A cohabitation agreement can be as simple or detailed as you choose to make it.
If you have children together, you should talk with your attorney about what kinds of matters regarding the children can be outlined in a cohabitation agreement or other legal document. Matters like child support and child custody can't be included in prenuptial agreements, for example.
While you're at it, it may be a good idea to develop some estate planning documents -- particularly if you want your significant other to have a say in managing your finances and/or health care decisions should you become incapacitated.
You can also designate what assets you each want the other to have should you pass away. If you have no legal relationship, you aren't each other's heirs under the law unless you designate each other as such.
You may not want to think about these things during your first days or even weeks of living together. However, if your living arrangement turns into a long-term one, it may be a good idea to seek legal guidance to draw up a cohabitation agreement.