My brother tried to apply for French nationality after his 18th birthday before going to university, but the application process is so slow that he wasn't interviewed until several years into his studies. The French immigration officials didn't like that he wasn't living there, so they deferred his application until he returned (and he chose not to return, but went to Australia instead). It may have also been complicated by going to the International School of Geneva in Switzerland, rather than a local French school.
I don't have any other siblings born in France, and my mum is past the age of childbearing unfortunately! My parents were able to naturalise and become French after 34 years. They applied when my dad retired, and after about 2 years they finally got it, in the midst of the political troubles of the UK leaving the EU. There was some worry about whether my parents would even be allowed to continue living in the house they spent 25 years paying off a mortgage to own. They couldn't apply earlier because CERN is an international organisation and doesn't pay French tax, so my dad was only paying French tax after his retirement.
If you have lived in France for the past 5 years (and still live there) you can get French citizenship. If you have obtained a diploma in a French university, you only need to have lived in France for the past 2 years.
If you have moved since then and don't live in France anymore, it might be too late though (unless you move back to France and wait 2 or 5 years).
(Also if you have siblings with French nationality, maybe if they were born in France after you were born in Switzerland, you are already technically French but have to claim your nationality).