When Ron is seven years old, he is given an acid pop by Fred. The sweet burns a hole through Ron’s tongue. Molly punishes Fred by hitting him with her broomstick, according to Ron’s recollection (PA10).
All we know is that Ron was seven-years-old when this happened (PA10). Counting forward from Ron's birth on 1 March, 1980, this incident most likely occurred at some point in 1987.