Daphne is frequently interpreted as having a strained relationship with her parents, but also coming from wealth, and being able to rely on that wealth to some degree.
A minor can be added as an 'authorized user' to a parent's credit card, but cannot have their own until age 18 (raised to 21 without cosigner in 2009).
It was illegal for women to have their own credit cards without a male cosigner (parent or spouse) until 1974.
Daphne wants a credit card, that she can pay for from her trust fund, or from money made solving mysteries, that her parents can't actually look at and see purchases on.
Daphne marries Fred for Legal Lending the second they turn 18 in like 1970, along with a host of other "Daphne is the money manager for the Mystery Gang but isn't legally allowed financial independence, so Fred just cosigns everything for her and then lets her do whatever she thinks is best" reasons.
From the “Scoobynatural” episode of the hit CW show “Supernatural,” Daphne Blake in her night gown. Consulting some old 1969 Sears, JC Penny and Montgomery Ward catalogs, I was able to rough out this costume change.
daniel got tired so i got cozy. i don’t think i make a good pillow but whatever. if he doesn’t get up in 5 minutes i might shove him i can’t feel my leg