How likely is it for a 20 year old person without a spleen live till 80? Percentage or chances? In regards to life expectancy or cancer or such? Thanks

Very good chance. When a spleen is removed there is a high incidence of infection and pneumonia for which the pneumococcal vaccine is given. If the spleen was removed due to a nonmalignant condition then the complications of splenectomy are markedly reduced since the cases where problems can arise are when the procedure was done for enlarged spleen secondary to lymphoma or Hodgkins.
As for anybody. Your not having a spleen, whether it was removed due to trauma or illness, will not diminish your life expectancy as long as you're alert to possible bacterial infection of the bloodstream and get seen if you get even a mild unexplained fever. I trust you've had the pneumococcal vaccine. There's no increased cancer risk. Ignore the scare-stuff online; it's just not true.
Totally possible. Although you have less protection against encapsulated bacteria, modern medicine has made it possible to live you full life expectancy although no one can guarantee that anyone can live till 80. Just be sure that your doctor pays particular attention to vaccinations and follow-up.
Having no spleen. is common, many people don't, either born without, lose it to a disease or trauma and get surgically removed, and other conditions, vaccines against pneumococcus, meningitis and h. Flu should be administered, others like flu shots every year would be needed, antibiotics should be started as soon as infection is suspected, keep antibiotics handy when you travel, follow up with doctor often,