Age discrimination dutch company

I (27F) work for a Dutch company (in Maastricht) for closely to two years now, and with the end of the year evaluation I asked that my salary would match with the other people on my team. This was because, it came to my attention that some worked only 4 days a week and had a higher salary. The main excuse they presented, was because they are older than me and therefore are entitled to more compensation. Mind you, we all started working around the same time and one of them actually started after me. Does this make sense? I mean, I didn't even asked for that much, just to have the same compensation since we do the same work (same tittle) but I'm full-time. And they told me that my salary was good for my age. They say its pretty common in most companies, is this normalized?