I think your dad might have misunderstood this (image attached) label perhaps? I used to work in a car seat shop and this must be the worst designed label in history because it was CONSTANTLY being misinterpreted by people! I cannot understand how it got past whatever design committees they have. It's an international standard and needs to be understood without reliance on text, but so many people pointed to it and mentioned something that made it clear they had assumed it meant the complete opposite, or some kind of variation on the meaning. Most often they assumed that the entire seat, or the rear facing mode was only suitable up to 15 months, which is totally backwards.
What it means is that babies age 0-15 months are not allowed to be forward facing in this seat by law. This is part of the R129 standard aimed at keeping children rear facing longer. To a minimum of 15 months, but preferably longer than this.
The vast vast majority of R129 car seats bearing this label can accommodate children rear facing much longer - usually up to 105cm which is about age 4.
I would apologise for being angry since I think this is a genuine misunderstanding, and they are trying to use the seat according to the instructions, which is important. Then I'd ask your parents if you can look at the seat manual together to check. I would bet that it says you can keep rear facing longer, most of the manuals actually point out that it's safer and encourage longer use of the rear facing position. I have not come across any R129 seat which has a rear facing limit of 15 months, aside from infant carriers - but infant carriers don't have this label. This label only appears on seats which have the ability to be fitted forward facing, even if they can be rear facing as well.
If they have lost the manual and you can't get hold of a copy, there will be instruction stickers on the side of the seat which show how it should be fitted for different height categories, so you could check this.
Unfortunately Halfords staff frequently can't seem to tell one end of a car seat from the other, so I'd definitely trust the manual over whatever a random sales assistant in store said, even if they all said the same thing.