I believe the answer to this question needs to come from both your heart and your brain and needs to be asked of both people before taking this step.
Here are the first set of questions: Do I love/deeply care for this person?; Are we both ready for a long-term committed relationship?; Do we communicate in a healthy and respectful manner?; Are we able to work out our disagreements and come to positive solutions that take into account both our unique personalities?; Are we willing to make this relationship a priority and work hard to make it a good one?; What are our long-term expectations for this relationship?; and Is my partner willing to talk things out, show respect and make compromises in difficult situations?
Here are the practical questions to ask each other, answer and put down in writing in case things do not work out: Who will pay for the rent/mortgage or how will it be divided?; Who will pay for which utilities?; Whose name (thus ultimate responsibility) will be on the bills?; Can we agree to where we will move, set up the household so that it fits our budget?; and What happens if one person decides to move out so that it is fair for both people?
If you can answer all these questions and still feel good about the move then your relationship is mature enough to take this next big step.