Thank you for your question.
This question is not so much whether it can be done rather than should it be done. Engaging in sexual relations is a big responsibility that have both mental and physical implications and consequences.
At age 15, a person hasn't matured to a level to accept all that can come with engaging in adult intimacy.
As for the 19 year old, it is wrong that an adult would be engaging with a child , which is what the 15 year old is still considered.
I hope this helps.