I found the following sources that shed some light on your question:

“that man, as a spirit, was begotten and born of heavenly parents, and reared to maturity in the eternal mansions of the Father” First Presidency Statement: 1909
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Orson Pratt taught:

If we were born in heaven before this world was made, the question might arise as to the nature of that birth. Was it by command that the spiritual substance, scattered through space, was miraculously brought together, and organized into a spiritual form, and called a spirit? is that the way we were born? Is that the way that Jesus, the firstborn of every creature, was brought into existence? Oh no; we were all born there after the same manner that we are here, that is to say, every person that had an existence before he came here had a literal father and a literal mother, a personal father and a personal mother; hence the Apostle Paul, in speaking to the heathen at Ephesus, says, “We are his offspring.” JD 15:246
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Ella Jensen, while in the spirit world, saw children:

…another room filled with children. They were all arranged in perfect order, the smallest ones first, then larger ones, according to age and size…They seemed to be convened in a sort of Primary or Sunday School…

Life Everlasting, Duane S. Crowther, p. 174

I’ll gather together a few quotes that can be used to support the idea we were born as children and then grew. I don’t think we have anything conclusive.

I would also like to have your opinion–if you believe the above–on how extensive you felt the pre earth life was, ie. did we organize into family units, make covenants, etc. Obviously the eternal principle of gaining light and knowledge, and having greater opportunities and advantages in the next world through diligence and obedience in a preceding world, would translate into our opportunities here, but to what extent, and how would that relate to the doctrine of omniscience?
I would love to hear your comments. Dan

PS I Geoff J is right to an extent, spirit is eternal but I believe that we were organized–added upon by God– from intelligent matter into a spiritual being and therefor became a begotten son or daughter of God. The question again is was that spirit that was then organized, an infant type or a adult type individual or entity?

Thanks for your thoughts and the link.

My purpose in posting a brief outline regarding permortality is to touch on the subject with the hope of increasing faith in the gospel of Jesus Christ. Premortality, in my opinion,is an essential point for followers of Christ to understand. It helps answer many questions for the thoughtful seeker. However, the essential points of the doctrine is all that is needed to increase faith.