To summarize what I said before, a mature account in a casino is when you are in the VIP program and if you can receive weekly and monthly bonuses it is better.
It's definitely nice to have an account at the casinos that is in the VIP program and can take advantage of higher promotions and rakeback programs. If I had to choose between having 4-5 normal accounts at different casinos or one older account with VIP status at one casino I would always prefer the higher rated account. Given that it's a well established casino that is going to be around for a long period of time. The earning potential of higher ranked accounts is very different from any beginner account, there are a lot of different promotions only for VIPs which we don't have any access to as a standard account. It's good if we reach such a status during our gambling career, but I wouldn't only gamble with the sole purpose of reaching a VIP status. As a regular gambler these things come by playing a lot over a long period of time and we shouldn't be too concerned about the status, because it can lead to us betting above our means only to try and reach a higher status.