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Childhood Sweetheart from WikiPedia:

Childhood sweetheart - is a reciprocating phrase for a relationship (but not a partnership) between young persons, traditionally of the opposite sex (Girlfriend / Boyfriend) occurring in their formative years (school age). This may come about by an extension of friendship, physical attraction or develop from natural affinity.

The relationship is usually platonic and lasts a short to medium period of time. This experience forms the basis of subsequent future relationships later in childhood and/or adulthood. Usually, an individual will have up to only one childhood sweetheart as this term is indicative of a milestone in the growth, development and maturity of a young person. In ideal circumstances, the term applies mutually to both parties and corresponds both ways, hence the plural being childhood sweethearts.

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