No, it's not immature to believe in "soul mates". However it is immature to believe in a "sole mate" such as thinking out of a world that contains more than 7 Billion people there is only ONE person who you could have a special connection with.
The concept of "the one" is based upon our natural tendency to (exclude) rather than (include) when it comes to finding love.
For example if a person states "the one" has to be a member of their own race that automatically eliminates (Billions of people) right there! If you went on to say he/she must have the same religious belief that will cut down your options by several billions more.
We narrow things further by stating "the one" must reside in our own country, state or town. All of this is before we get to height, weight, age, occupation, education, hobbies/interest, and goals....etc Last but not least our family and friends MUST also like him/her and vice versa!
It’s no wonder that by the time we get done EXCLUDING people there is ONLY one "right one" left!
Assume you are looking for someone who has the following traits.
Attractive, Positive, Affectionate, Considerate, Loving, Healthy, Romantic, Passionate, Great Sense of Humor, Intelligent, enjoys traveling to exotic destinations, Honest, Trustworthy, Loyal, Dependable, Financially secure/responsible…etc
Surly out of 7 Billion people there must be “one” person who fits this profile! I bet there are (millions) of people that identify as such!
No one is going to be able to read your mind, complete your sentences, or fill your days with sunshine for eternity. You determine how large your pool of potential mates is going to be by your method of (including or excluding). Naturally the fewer options (you allow yourself) the more difficult it becomes to find a "suitable mate".
“Love isn’t finding a perfect person. It’s seeing an imperfect person perfectly.” – Sam Keen