They declared their feelings for each other sometime in S1 (after Jitters), but they didn't consummate their love* until after Clark either (a) turned 18, or (b) graduated from high school (since high school in the SV-verse starts at 15/16 and ends at 19/20).

*This was due to a variety of factors -- Clark's age, Bo's shotgun, and Lex's inability to believe that he was capable of loving someone, to name a few.