Perfection
The Great Head.
HOW WILL WE DESCRIBE PEOPLE FROM AGES 13-19 THEN? 

Because ten is the correct way to say one when added to another digit? It would be tenyone, tenytwo, tenythree. Instead they put the ten in the reverse order and you ended up with teen. Eleven and twelve are probably left overs of a 12 base system, that had 12 unique identifiers.
11 and 12 are holdovers from a base ten system for Germanic farmers. They translate roughly to "one more" and "two more" than the ten (fingers)
You are probably right, but 12 has more significance than just two more than ten. I am probably off in that there was a base 12 system, but it would seem historically, that 12 was a defining point even outside of language. 12 inches does define an english foot. Even the pre-germanic languages though had specific terms for eleven and twelve.
If we were to follow the Welsh example then we would be say ten one, ten two and so on, not what the OP described.
Is that 80% of the vocabulary is Chinese, or the most commonly used words are Chinese?
Wikipedia said:Languages using duodecimal number systems are uncommon. Languages in the Nigerian Middle Belt such as Janji, Gbiri-Niragu (Gure-Kahugu), Piti, and the Nimbia dialect of Gwandara;2 the Chepang language of Nepal3 and the Mahl language of Minicoy Island in India are known to use duodecimal numerals. In fiction, J. R. R. Tolkien's Elvish languages use a hybrid decimalduodecimal system, primarily decimal but with special names for multiples of six.
Germanic languages have special words for 11 and 12, such as eleven and twelve in English, which are often misinterpreted as vestiges of a duodecimal system.citation needed However, they are considered to come from Proto-Germanic *ainlif and *twalif (respectively one left and two left), both of which were decimal.4
Historically, units of time in many civilizations are duodecimal. There are twelve signs of the zodiac, twelve months in a year, and the Babylonians had twelve hours in a day (although at some point this was changed to 24). Traditional Chinese calendars, clocks, and compasses are based on the twelve Earthly Branches. There are 12 inches in an imperial foot, 12 ounces in a troy pound, 12 old British pence in a shilling, 24 (12×2) hours in a day, and many other items counted by the dozen, gross (144, square of 12) or great gross (1728, cube of 12). The Romans used a fraction system based on 12, including the uncia which became both the English words ounce and inch. Pre-decimalisation, Ireland and the United Kingdom used a mixed duodecimal-vigesimal currency system (12 pence = 1 shilling, 20 shillings or 240 pence to the pound sterling or Irish pound), and Charlemagne established a monetary system that also had a mixed base of twelve and twenty, the remnants of which persist in many places.
The importance of 12 has been attributed to the number of lunar cycles in a year, and also to the fact that humans have 12 finger bones (phalanges) on one hand (three on each of four fingers).5 It is possible to count to 12 with your thumb acting as a pointer, touching each finger bone in turn. A traditional finger counting system still in use in many regions of Asia works in this way, and could help to explain the occurrence of numeral systems based on 12 and 60 besides those based on 10, 20 and 5. In this system, the one (usually right) hand counts repeatedly to 12, displaying the number of iterations on the other (usually left), until five dozens, i. e. the 60, are full.67
A related hypothesis is that numbers in Chinese (and related languages) are much shorter to say.
I doubt there was a base-12 numbering system. Numbering systems in language for the most part develop into one of four systems:
That is literally what "thirteen, fourteen, fifteen, sixteen, seventeen, eighteen, and nineteen" mean.
It's because the word "Ten" is singular while "twenty, thirty, fourty, etc." are all plural. So it doesn't make sense to say Ten one, but it does make sense to say Twenty one.
I'm probably wrong, but it's probably something like that. The same thing is done in German and Polish, hinting at this convention existing for quite a while..
(Another way to think about it is that Ten is one ten, while twenty is two tens.)
Saying Teny one seems reasonable. Someone suggested that earlier.
I thought you were Bulgarian?
Yeah, but "Ten" is a reference to the "Tens" part of the number.
Ten = one ten
Twenty = two tens
Thirty = three teens
...
Teny sort of looks like it might imply that it's a plural amount of tens. It also sounds too close to "Twenty"