There is no definitive answer to this question as Tails' age is never specifically stated in the Sonic the Hedgehog canon. However, considering that Sonic Mania takes place after the events of Sonic & Knuckles, it is safe to assume that Tails is at least a few years older than he was in that game. Based on this, it is reasonable to estimate that Tails is between 8 and 10 years old in Sonic Mania.