Zor-El had a teen age daughter while his brother had bupkis.My guess is that since those actors would be interacting with the adult actor playing Clark (holograms, ghosts, anything really) as well, they chose older actors so that they'd still look believable as his parents compared to Adult Clark.
It's the same reasoning used in Harry Potter. His parents were only barely adults when they died (around 20) but when Harry sees them as ghosts or as magical images they look in their late 30s or so because they wanted them to look believable as the parents to a 12 year old and later teenager.
Was Zor-El younger or older than Jor-El...
Do I recall a plot line from Smallville where Zor-El wanted to marry his sister in law Lara?