Any sexual contact between a 15 year old and an 18 year old is illegal. See Texas Penal Code Section 22.011 - Sexual Assault and Section 21.11 - Indecency with a Child. Age differential MAY be an issue, but it is best not to go through the process, Wait.

Can a 16 year old be with an 18 year old Texas?

The age of consent in Texas is 17. This means any individual 17 years or older can legally agree to have sex or participate in sexual activities. Therefore, any minor 16 or younger cannot consent to sex with adults.