Tommee Tippee teats are staged as slow flow 0 month+, medium flow 3 month + and fast flow 6 month +. These age stages are however just for guidance. You can usually tell when the baby is ready for the next stage and it may happen quicker than expected. If your baby is comfortable with their current flow rate then be rest assured that you do not have to change.
If you find the milk flow using the slow flow teat is too fast, you might want to consider variflow teats, which have an X cut at the end allowing your baby to dictate flow rate. Your baby may also be ready to move on to a sippy cup, such as our first sips cup.
No matter what flow rate your baby is comfortable with, you should replace all your teats regularly to ensure they are in good condition, Tommee Tippee recommend every 8-weeks for variflow whilst every 12-weeks for easi-vent. The bottles themselves will last until baby is fully weaned off the bottle.