There are several things to consider in this question. First of all, let’s define “Christian.” Many people assume they are Christians simply because they are not affiliated with any other religion. They go to church and agree with most of what the Bible says. However, the Bible defines a Christian as a disciple, or follower, of the Lord Jesus Christ (Acts 11:26). A Christian is someone who has accepted the death, burial, and resurrection of the Lord Jesus Christ as the payment for his or her own sin (John 1:12; Acts 16:31). A disciple of Christ has chosen to "deny himself, take up his cross daily, and follow" Jesus (Luke 9:23). Therefore, whatever Jesus says to do through His Word, a Christian seeks to do. We do not become Christians by doing good things; but, because we are Christians, we want to obey Jesus in all things (Ephesians 2:8-9; James 2:26). In John 15:14, Jesus said, "You are my friends if you do what I command you."
So a Christian makes life choices based on what glorifies Jesus (1 Corinthians 10:31). Better than asking whether a situation is "allowable" is asking "How will this honor my Lord?" God created marriage, and it is His definition we should use as our foundation. God defines marriage as a lifelong relationship in which a man leaves his father and mother and unites with his wife. The two become “one flesh,” and the union must not be dissolved by human will (Genesis 2:24; Mark 10:7-9; Ephesians 5:31). Malachi 2:14 tells us that one reason God hates divorce is that He is present when a couple takes the vows. Biblically, marriage is the joining of a man and a woman in a spiritual and physical covenant for life. That joining is cause for celebration and deserves our respect.
Some couples today, particularly among seniors, want to cohabit as “married” couples without being legally married. Often, this is done for some perceived financial benefit or for simplicity’s sake. Some of these couples undergo a religious ceremony in a church and consider themselves married before God. However, a couple seeking a "spiritual marriage" while avoiding a legal marriage is seeking to escape the requirements of the law, and that causes a new set of problems for the Christian (Romans 13:1-7). If a senior couple believes it is God’s will for them to be together, they should marry in accordance with the laws of the land, and trust God for the finances.
There is no scriptural basis for a live-in situation, even when the two involved intend to be monogamous for life. Intentions fail, and the lack of a real marriage commitment makes it easier to part ways. Without marriage, the relationship is sexually immoral and is condemned in Scripture (Galatians 5:19; Ephesians 5:3; 1 Thessalonians 4:2). The term "life partner" has a tentative sound and a questionable history. It implies that the relationship is not legally or morally sanctioned and that it may not last. It bypasses the covenant that God created marriage to be. For a Christian couple, such a term would cast immediate suspicion on their reputation and, ultimately on Christ’s reputation. Any Christian couple considering a “life partnership” should ask, “How will our bypassing of traditional marriage glorify the Lord Jesus?”