If an older person describes a younger person as sassy, they mean that they are disrespectful in a lively, confident way. Confidence & self-assurance (definition of sassy from the cambridge advanced learner's dictionary & thesaurus Β© cambridge university press) The meaning of sassy is having or showing a rude lack of respect :

Definition of sassy adjective in oxford advanced learner's dictionary. Meaning, pronunciation, picture, example sentences, grammar, usage notes, synonyms and more. Disrespectful or impudent; See examples of sassy used in a sentence.