No worries about your age at all! Mature students are actually quite common in Canada at all stages of post-secondary education, and even then I don't think you would even qualify as a mature student because you're under 25 and have been in school recently. You'd be considered just a regular, general track student. I have tons of lawyer friends and family who have entered law school in their late 20's and early 30's and have been very successful.
It might be different in the Netherlands, but here it's definitely not that big of a deal!
It definitely varies depending on the type of firm. Make sure you understand their practice area and the type of matters they handle (and don't handle). You could also look for any recent cases, which could also give you insight into the type of matters they handle beyond what is posted on their website. You definitely want to convey why you want to work for that firm, rather than the firm just being a stepping stone to something you're more interested in if the larger full-service firms didn't work out for you.
"Fit" is very important at smaller firms. They will mostly want to know if they can or want to work with you since you will spend more time in close quarters with everyone. You will likely have more direct client interactions at smaller firms, so professionalism and a level of maturity/common sense are also important. This is especially true if you are helping with intake calls and you are the first point of contact a potential client has with the firm before scheduling a consultation.
I can only speak for my firm, but these are the top few things we look for:
strong research and writing (we need help with written submissions, appeals, etc.);
relevant courses/knowledge of our practice areas (if you have no concept of the legal issues or type of matters we handle, we're not going to be a good fit);
personality/fit since we are a small outfit (we are a quirky bunch);
common sense/maturity (don't tell clients how intimidating the hearing was while you're on a break or forget to ask basic intake questions while on a call
I also think that smaller firms want you to interview well. They may not be interviewing as many students and have less time to spare on the process. We would typically receive 50+ applications, do phone interviews for the top 8-10 and then only do in-person interviews for the top 4 or 5. Relax knowing that they probably think you're capable of doing the job and that it's just a matter of whether you're the right fit for them or whether they have questions about your application that they want clarification on.
If you want to share the practice area without identifying the firm (or yourself) you may get better insight.
I knew a handful of foreign law grads in law school, all substantially older than 24. Most were in their 30s or 40s. Pretty sure median age for law students in Canada is around 25 (at least it used to be).
So the age or the past isn’t an issue. Maybe it’ll help you write a better personal statement too.