My first question is that what have I (possibly) lost by not networking?
Secondly, what is meant by "networking" exactly in an academic setting? Just knowing each other exists, or being some kind of friends, or just knowing who is interested in what?
These two questions are related, since what you've lost obviously depends on how you define networking.
In my view, networking is simply knowing and maintaining friendly relationships with people in your professional circle. It's nothing magical, it's just professional friendship. Those in your network may also be "social" friends, or they might be people you only ever see (or would ever want to see) in a professional context. Your network are those people who are more likely to take a biased and positive view towards you and your work, and for whom you also maintain a positive view of them and their work.
Thus, what you lose by not networking, is you lose a pool of people who might be willing to work together with you in the future, you lose those who might favorably review your grant application or journal submission, who might be willing to recommend you for a job or let you know about job opportunities in their orbit. On the flip side, you also miss out on having a group of talented people whose abilities and personalities you know and who you could recommend to fill a position in your own group or in your organization.
Thirdly, how can a graduate student network in events where most of the people are way older, and academically more senior?
I would say in this case focus on networking with your peers or close-peers (i.e. other students and postdocs). Networking is usually not about knowing people who are more senior than you, who have a ton more power than you, etc. Those relationships are typically more mentor/mentee, promotor/promotee. Your network are more likely to be made up of your peers. The people who are most likely to be in a position to benefit you in the future are your peers today, precisely because your relationship is not built on a transactional foundation. Those in positions above you in power/seniority are more likely to view things in what you are going to bring to them today/tomorrow, and already know enough people in their peer network that they're not in dire need of making a connection with you. Other answers about giving talks, asking questions and getting face-time with those above you at conferences is a good way to connect with those folks, but I don't think this will necessarily build a long-term network.
In terms of building up a solid peer network as a graduate student, you should start local. Take part in your school's student chapter of your professional society, take part in department events and interact with your group members. Beyond that, attend conferences as your schedule and advisor's budget allow. At conferences and other social events, don't travel just in a "pod" with your own group-mates. Take a chance to sit at a random table and introduce yourself around. If you see a talk from a student that you were really impressed by, tell them so and introduce yourself. You will probably be seeing those same folks at conferences for the next several years, so after the first meeting it will be more natural to grab lunch/dinner/drinks/coffee at future meetings.
Many of these people may not be those you have a strong personal connection with, but some will. For those whose company you enjoy, make an effort to stay connected between conferences and stay current on their career path. If in the future you're in a position to help their career, do so, and trust that they will do the same for you.