The short answer is: in many cases, yes (but).
Junior roles are just that: positions of a low rank in the corporate org structure, often filled by people with little-to-no professional experience. In situations where opinions differ (and sometimes other situations as well), your input will likely carry less weight for these reasons.
That said, a good senior employee or team lead will still take input from all sources, whether to provide feedback and coaching to junior employees, or simply for the practical reason that good ideas can come from anyone.
Your situation sounds more complicated
It's tough to get into too great of detail based solely on the information you've provided, but just from that it seems there are several layers to parse here.
1, Ageism is a real thing.
In tech, this often cuts the other way—discrimination against older people in tech is very common; it is commonly seen as "a young person's game." On the other hand, being young can definitely cause others to ascribe less weight to your input, especially if you are noticeably younger than your immediate peers/the median age of the company.
2. Being present is important.
The fact that you are part-time is definitely a factor here. Many things become orders of magnitude more complicated or difficult when you and your colleagues are not co-located. Not being around will dilute your impact. Period. This is true for people working remotely, but may be especially true in your case; your full-time colleagues may not take you as seriously because you literally aren't putting in the same work that they are. (Not that your work is less valuable or of a different quality, but by sheer quantity, you are by definition doing less)
3. You're new.
If most of the other employees have been there for years (5 years is a long time!) they likely have pre-existing relationships with each other and/or processes at the company. Having only been there a few months is not going to seem like much compared to that history,
4. Your autism may be complicating things.
You mentioned autism in your question. I don't know the details of your personal situation—that label covers a lot of territory—and I don't want to make assumptions. However, broadly speaking, social cues and interpersonal communication can often be more difficult for people with autism, and while I don't want to necessarily ascribe your difficulties to that, it may well be a complicating factor on everything else.
5. Your hiring manager isn't your manager.
You were expecting the person who interviewed you to be your boss, and they're not. Let this be a learning moment for you: when interviewing, ask who you will be reporting to. It sounds like you had a good rapport with this person, but not so much with your current manager.
What you can do about it
Don't just give up. You can still make this situation work for you.
Communication is king
Talk to your current manager. Try to avoid being confrontational about it, but share how you've been feeling ignored. Ask about the recent move to adopt that framework based on the consultant's input, and what was different about the current situation than when you suggested it earlier.
There may be broader context or other aspects to the situation that you are not privy to because of your junior role.
Again, I suggest being cautious about tone in this kind of discussion. It may help (especially if you do have difficulties with communication) to write down what you want to say in advance. I recommend having a friend read it over as well; an impartial third party will be able to tell you how things may be recieved.
Find mentorship
Consider talking to the person who interviewed you. Even in an informal context, getting advice from this person will probably be helpful. They may have advice about how to deal with the specific personalities and/or priorities on your team.
Be a little careful with this one—you don't want to seem to be going over your current manager's head. Depending on the culture of your workplace, there could be political ramifications to consider.
Adjust your expectations
You are a young part-time employee in a junior role. Your teammates should listen to you because you are a part of the team, but that doesn't mean that they'll listen to or accept everything you say. Nor should they. Do keep trying to be helpful and pro-active. Even if your main job duties are to write code, if you have input on or insight into architecture, frameworks, tools, workflows or the like, then share them! Helping the company succeed is also part of your job.
But maybe temper your expectations a little. It's ok to give input. It's not ok to expect that input to be taken every time (or even ever!) especially given the stage of your career.
If it's really not working out, it's ok to quit
You're young and you're a student. Give it your best, but remember that your career is only beginning. You don't have to be tied to this job. You've experienced for yourself a different workplace that felt better (your internship) so you know that this isn't "just how it is everywhere." Do what's right for you.
