I don't rightly remember when I first caught the language bug, but it must have been around 6th grade (1945-6), when I was 11-12. My sister had just started high school (9th grade) and was taking Spanish. She taught me how to count to ten in Spanish; then, since she seldom looked at it, I began to read through her Spanish grammar book. Then I discovered a set of old encyclopedias (vintage 1900) in our attic. At the onset of puberty, I was probably looking up things like "penis" etc., and maybe flipped some pages over to "Prakrit", which had a lot of cross-refs., which led to "Sanskrit" and finally to "Indo-Germanic" (as it was called in those days…), with more cross-refs.
Then in 7th grade, for some achievement or other, I was awarded one of those silk things that American pilots in Asia sewed onto their jackets—it said "I am an American pilot. My plane has crashed in your country. If you help me avoid capture, my government will reward you" or some such, with translations in Chinese, Thai, Vietnamese and French (!). That's when I fell in love with Thai script.
Then I discovered on our bookshelves my mother's 1920s college English grammar, and learned that English had a subjunctive (if I be, if you be, if it be etc.). Next I found her old college Latin grammar that absolutely fascinated me. Based on what I could figure out about Sanskrit and Latin, I began to invent a language, naturally called Millsaic. It consisted entirely of one verb, bharanoi 'to bear' with several pages of conjugations— indicative, subjunctive and I think desiderative moods; present, past, future tenses; perfect tenses; imperatives-- all with their own endings. This must have been during 8th-9th grades, because I know it was done when I went to boarding school in 1949 for 10th grade (it was called Fourth Form at the school), and would have friends quiz me on the language—"what's the pluperfect, desiderative mood, 3rd person?" and amazingly I knew it all by heart. But I tired of it, and Millsaic never got beyond that one verb. In that year I made a good friend of another boy who'd been to school in England and had also invented a Romance-type language, with a translation of Carroll's Through the Looking Glass called Per lo mirroro. He became my roommate in the next (Fifth Form) year.
Then in the summer of 1950 I invented Thenian, whose name derived from its principal deity Itha Theni. I devised a script that was basically a syllabary, but also had signs for single vowels as well as one- and two-syllable signs (e.g. –nigi, which was the dative case ending). I wrote a lot of sort-of religious texts in it; I remember nothing of the dogma, and only remember the beginning of one prayer, a take on the Latin Gloria (I'd also become acquainted with the Latin Mass)—
Munane Itha Theno, fekerud inekadrud mhundei iminane deniei mighty I. T.-Voc creator preserver world-GEN all thing-GEN O mighty I.T., creator and preserver of the world and of all things…
From this tiny bit you can see that it was a mixture of Latin-based and totally a-priori stuff; the vocative ending was –o, the nominative was –ud, the genitive was –ei, plus the aforementioned dative –nigi. My roommate and I actually used the language in our occasional evening prayers. But that was the end on my early conlanging years.
Next came college, the Army, various jobs, finally graduate school in linguistics at the University of Michigan, a doctorate in 1975 and an ill-fated teaching job in 1975-6. The rest is told in my little essay "Birth of a Planet (and three languages)" that appeared in Fiat Lingua: http://fiatlingua.org/wp-content/uploads/2011/12/fl-000004-00.pdf .