Your question was:

>How would I be able to explain the Trinity to a 15 year old?

Of course you understand that the word "Trinity" is not in the Bible, but is a term coined by men to explain the nature of the Godhead.

The Bible teaches there is one deity (God) who has revealed Himself in 3 persons: God the Father, God the Son and God the Holy Spirit. Jesus said "I and the Father are one", not because they are the same individual, but because they are both divine in nature (cf. Jno. 17:5; 1:1-3, 14).

I suppose I would liken the Godhead to the unity of marriage: the husband and the wife are two individuals who, as a result of being joined by God, are "one" (Matt. 19:6). They are separate, but they are united. Even so, God is a "united one".

Also, I would use John 17:20-21 in which Jesus prayed that believers would be "one" in the same way that He and the Father are "one". Believers can be united even though they are different individuals. God (deity) is united (one) although composed of the Father, the Son and the Holy Spirit.

I hope this helps. Thank you very much for your question.

Sincerely,