Can you lean to sing in your 20s or 30s

The simple answer is yes. You can practically learn to sing at any age. However when you are older you may need help of a professional teacher. If you have been singing since childhood then lot of basic things about singing are known. Like singing in correct pitch, correct volume , applying correct riffs and runs, being a performer and not just a singer etc.
Personally I learnt to sing in my 30s. When you learn singing in your later age you do not have to worry about your voice changing. voice generally undersgoes a change from Child --> Teen --- to ---> Adult transition . Lets see some of the advantages of learning to sing at older age.
1. You know your limitations and you are not trying every thing.
2. You know your voice quality and voice pitch, and probably even the songs that suit you.
3. You will have more discipline during practice. No one has to push you.
4. You can better control your diet and your voice. At young age there is a tendency to break rules and eat and drink things which are not good for singers.
Lets see what are the disadvantages of old age
1. Attitude problem and I know it all can come in the way of your learning. You may argue with your teacher and that may prove to be fruitless
2. Set mindset about your limitations may prevent you from learning. 
3. 30s to 40s people are more choosy and not willing to bend. This can make it difficult for you teacher. 
4. If you are caught up with too many things, you may not be able to learn at older age. 
Remember singing is a skill and you can learn the skill at any age. There is no age limitation , you just need to be flexible, have open mind, unlearn things , hire a professional teacher. But above all you will need passion and drive and never say die attitude.