What is an appropriate age for a promise ring?

The reason I ask is my step-daughters boyfriend has recently asked to give her one. He asked me through a message (and apparently I didn’t reply fast enough because he blew up my phone). She is only 14. I asked her dad what he thought (the reason it took me a little while to respond) and he and I both said she was too young for that kind of commitment. Which she apparently knew he was going to get her one. So she’ll probably flip out when she finds out her daddy said no. They have been on and off 3 times for about 8 months. All times he broke up with her. Last time split for 1 month and he posted single AF on his social media. Which she denies (seen screenshot on her phone). Also denies that he was trying to date another girl immediately after his single AF. Now they are all Lovey dovey back together for not even a month. Emotions are running too high for this 14 year old. But you can’t tell her anything. I don’t really like promise rings. I feel they are some what a waste of money. Because really you should save your money and buy a real thing when you’re older and understand what love really is. I don’t think any age below 18 is good enough for a promise ring. Talk of marriage and making a commitment like that is a big deal. Not a 14 year old big deal. Even if it’s the promise of one day getting engaged. Right now it more seems like there is another motive for this ring. Which if he still gives it I’m afraid her dad is going to have a fit! 😱

She is definitely not mature enough for it and I don’t believe the boy is either.