It happened around 3:50 p.m. Thursday on the unit block of Tall Tree Lane.
Authorities say the suspect was armed with a sharp object and got away with the couple's cash.
Police were able to create a composite sketch with the help of one of the victims.
The suspect is described as a white male, between 30 and 40 years of age, 5'8" to 5'9"tall, who was wearing light colored clothing and a hat.
Anyone with information is asked to contact the New Castle County Police Department at 395-8192.
Tips can also texted to 847411 (start messages with NCCPD then type your message) or by calling Crime Stoppers at (800) TIP-3333.