So I know Smallville and canon go together like... not oil and water exactly, but two unmixy things. Still, you'd think the writers could remember the ages of their characters at least.
Julian Luthor/Grant Gabriel- This is probably the least annoying since it never technically contradicts canon. Grant isn't actually Julian, just a clone whose aging was sped up, so really he doesn't have to be the same age as Julian. It's worth noting though, that in S03E19: "Memoria" it's established that Julian is twelve years younger than Lex. This means that during Season 7 when Grant is running around, Julian would be about fourteen/fifteen. Again, Grant doesn't have to be the same age as Julian, but he thinks he is, so why doesn't he question the age discrepancy?
Davis Bloom- In S08E08: "Bloodline" Davis comes to Chloe and tells her that he can't find any information on the first three years of his life. Fair enough. The assumption then is that he came to Earth like Clark and wandered around until someone found him and guessed he was about three. But then in S08E18: "Eternal" we see lil' Davis right after he lands on Earth and no way anyone thinks that kid is three. Maybe six or seven, but definitely not three. ETA:It occurred to me somewhat belatedly to check this, Alex Ferris, who plays lil' Davis, would have been twelve at the time of filming, so yeah. I may have been remembering him as looking younger than he did to try to give them some credit. (Lil' Clark in the pilot, by the by, was played by a seven year-old. Clearly the casting department is having issues as well.)
Tess Mercer- In S08E16: "Turbulence" there's a newspaper article about Tess, which, if you pause and read it while it's onscreen, says she's thirty years old. Then, in S10E08: "Abandoned" we see Tess's birth certificate with her birthday on it and if you calculate her age from that she's... still thirty years old. I guess the newspaper article could have been wrong, or maybe the Mercers decided to make her two years older when they adopted her for some reason?
Oliver Queen- Between S06E05: "Reunion" and S07E15: "Veritas" Oliver is established as being about the same age as Lex, maybe a year older or younger. That means that during the first meteor shower Oliver was somewhere between the ages eight and ten. S01E01: "Pilot" established, via newspaper article, that Oliver's parents went missing/were declared dead right about the time of the first meteor shower. But in S09E04: "Echo" Oliver tells Clark he was five when he lost his parents. So either Oliver was lying, Lex was letting himself as a senior be bullied by a freshman, or the writers have no idea what they're doing.
Lucas Luthor/Dunleavy- This one!!! Lucas's age bothers me SO MUCH! Okay let's start with the fact that Rachel Dunleavy shows up in S02E07: "Lineage" and is convinced that Clark is her son. This would suggest that Lucas was the same age as Clark. The problem with this is the adoption agency that was used for Clark and Lucas's adoptions was only open for six months (according to Chloe in S01E14: "Zero") and since Lucas is heavily implied to have been given up when he was a baby, he would have to be three years younger than Clark to have been adopted out through the same agency. But this might be excusable, since Rachel was clearly insane and might not have even realized that Clark wasn't the right age. But then we have S02E15: "Prodigal" where Lucas actually shows up and is eighteen, two years older than Clark. Which means he had been given up for adoption through an agency five years before said agency actually existed. What in the world is going on here?