Didnt know it was necessary to change teats accordingly. My girl is 12mo now and still using newborn teats !
it's perfectly fine! my girl is 10mths old and still using 1mth old teats from pigeon..
my baby use newborn treat till she 4 months old I just change to level 2 1 month ago
I personally feel it's ok. My 10MO is still using no.1 teats.
Its fine. Give the teat that they can manage.