You can take that up now...... The particular training or education that you wanted as a child or teenager but could not... May be due to parents' or guardians' pressure or due to financial situation, etc,.

You wanted to pursue life sciences but you were pushed to computer science.

In this case life science was something your heart wanted and you still take a deep breath when you see people talking with great knowledge of life sciences.

What stops you from studying it now. Educate yourself. Your heart does not need to see a certificate from any university. It jus needs to know that you have learnt all that you were inquisitive about. Find out the current syllabus of the subject, educate yourself through books that will help you bridge the knowledge gap that has come due to the developments in all these years.... You can even start with the current secondary school books of your school... That will refresh you and act as a bridge.
You can stand in front of the mirror and say to the person inside proudly that you are a master of the subject now.
And next time you can contribute actively to the discussions rather than taking a sad deep breath.

1. Now you have time which you are spending on FB or fine tuning that presentation on weekends. Do it to enrich yourself