In Malay, belas is the suffix given to numerals to indicate the numbers 11 to 19, so 11 is sebelas, 12 is dua belas, and so on.
Then there is the Malay word belasan, to refer to the age group 11 to 19. My dictionary gives the gloss for belasan tahun as 'teens'.
However, this is not quite right, as in standard English, the teens only start at 13, and people aged 11 and 12 are not teenagers.
I wonder if there is a shift of meaning of the word 'teenager' in Brunei English, influenced by the Malay word belasan?
One further influence might be from Chinese, as there is no easy translation of 十几岁 ('aged between 11 and 19') in English. It seems possible, therefore, that this shift in the meaning of 'teenager' is found quite widely in the region, including in Singapore.