Since I remember I have always been inventing worlds, and I have always liked cryptic, and languages, as so many other stuff (Math, Physics, Computers...)
I can't remember any infant attempt to inventing a language. But when I was a teenager, I begun to see useful coming up with a way for writing down what I feel, who I feel it about, and so on. (see my 1988 scripts).
One first attempt to coding was based in a way for handwriting Morse code, making every symbol in a visually connected character. Dots would be represented by horizontal lines, dashes as vertical ones. Adjacent dots would be set one over the other, etc. This code was named EISH5.
EISH5 those days was very regular. Today's EISH5, though seldom used, allows some freedoms so one letter (lets say A: dot dash) could have many shapes so a human decoder, knowing nothing about EISH5 philosophy, wouldn't recognize it was the same letter. Look at example:
"Carlos" in Morse Code
"Carlos" in the original EISH5
"Carlos" in the new EISH5.
I had learnt the Greek alphabet (though not the Greek phonology, nor the Greek language). And used it sometimes, but it was an easier code for decoding than EISH5.
After learning the Cyrillic alphabet, I came out inventing a new alphabet. Based on Greek-Latin-Cyrillic style, borrowing some letters with the same or different value. This code was initially thought to crypt Spanish in a more complex way than a letter-by-letter substitution. The script was named after my self thompiniano (actually some class mates would have given it that name).
After that script I begun to create a language. It was a three or some years job which was never finished and never will. I have wanted to reconstruct that early Conlang but the files and papers are lost.
I begun to study Swedish, after I lived in Sweden a little while. My invented language was based in my own Ideas an influx by Spanish, English, Swedish and French. It was a "personal language", somehow a "fictional language". I was not an "auxiliary language" but something more like a ciphering code.
If you want to know a little more on these scripts and language follow the links bellow:
In order to enrich my mind, I found in a bookstore in Sweden a dictionary Esperanto-English with grammar and other self-study stuff. It seemed appealing to me to apply some ideas from Esperanto into my Language, as I previously did with Swedish. After some obvious flaws I saw in Esperanto (like opposites, and feminines), I began to think about inventing an International Auxiliary Language (IAL). I read a booklet on Ido, agreed with critics of Esperanto but didn't like the solution: too French, seamed to me. I found something on Interlingua, which I liked, and some other IALs.
I drew a lot of testing, both on ASCII and non ASCII charsets, including Thompinian. My first language (RI.T.H.) was not an IAL and some grammatical structures were not planed for simplicity and clarity.
When I came back to Colombia, and begun attending college I lost a lot of stuff and, little time after, a del *.zip command, not reversed by a undelete command, erased many of my backup files, including world and language invention.
Finding in Internet a lot of constructed languages, interest was renewed. I am subscribed to the Conlang list an have some active projects.