How to add a Rownum id for a set of data? SQL

Question

How can I add a Rownum id to a set of data, I want this id to be unique and when I make a new insert this Rownum will increase. SQL, PL/SQL

For example I have this data:

Name    Age
Anne    26
Mary    45

I want to add a Rownum id that starts from 1 and increase for each row. The result expected is

Id    Name     Age

1      Anne    26

2      Mary    45

When I insert a new row the id will be 3

Id    Name     Age

1      Anne    26

2      Mary    45

3      Jhon    41

I think I need to do the maximum of Rownum for that but I can't find the perfect SQL/ PL/SQL code.

SELECT NVL(MAX(ID),0) FROM TABLE;