How should I deal with a lower enlisted of above average age (late twenties) who constantly has disrespectful incidents?

Disrespect is contagious. This person's age has nothing to do with how to approach it. An NCO in her chain needs to address it ASAP. She should be put in check when she disrespects others regardless of rank, age or gender. She needs to be reminded that in the civilian sector she may be in management, in the military she needs to learn her place and obey lawful orders without being disrespectful or she needs to be processed and discharged before her attitude and demeanor become pervasive within the unit. Disrespect cannot and should not be tolerated.

Sounds more like she has issues with taking orders from younger folks in charge, every platoon Sergeant goes through something similar when he/she gets anew PL. Without more details about how she is being disrespectful and what orders she is failing to follow it is difficult to tailor a response. My old grouchy senior NCO response is to tell this troop to get her shit straight, follow the lawful orders of her NCO's. If that doesn't work and the sit down and discuss the issues method fails then document everything and she will be the oldest whatever her rank is that the Reserves or GUARD has ever had right before she is DFR'd. Leaders don't have enough time in the day to deal with squared away troops let alone deal with labor intensive drains on their time and effort. Its doubly important during a Mob to focus on the motion and not Divas. I would also reach out to your peers and higher in your NCO support chain for wisdom and methods to fix the this troop or remove her from the ranks.

The same way you deal with any Soldier. Age gains no additional benefit of the doubt. Lead, follow, or get the hell out of the way. Lead from a position of respect. Know yourself and know your Soldiers, and know your mission. Thank you for your service.