Is it forbidden to tell a girl "I love you" at the age of 19 and 18?

You are not permitted to tell the girl you love her, as this may cause fitna (temptation/strife). If you are able to marry, then propose to her guardian for her hand. Marriage is the solution for those who love each other, as the Prophet, peace and blessings be upon him, said: "There is nothing like marriage for two who love each other." The reason for the prohibition is that she is a non-mahram to you, not because you are both teenagers.