18 months is considered old for a hamster really - it's the equivalent of about 60 in human years
If the vet has checked him and said he seems healthy for his age, I assume he means there may have been some neurological damage as the result of a fall? Some vets don't know much about hamsters and that doesn't seem much of a diagnosis.
Regarding the levels - if there is 25cm between them then presumably the cage is at least 50cm tall. That is quite a drop for a hamster, from the top, particularly if they monkey bar across the roof bars at night. They can bounce off shelves on the way down as well. If you have lots of deep substrate in the cage, and make sure any hard toys are under a shelf, then it can cushion any falls, but it doesn't stop them getting caught on a shelf or landing on a hard ladder, when they fall.
Do you have a photo of your cage and we could suggest some tweaks to the set up or shelves to help prevent fall risks. Although my feeling is that if he has not been himself and he is getting older, it might be better to remove the levels or just have one low level. They can fall when they get older, either from weak back legs and losing balance or similar. It would be good to see your cage before changing anything though as changes can cause stress as well, so planning them out first can help.
Does he still look down and ill? He may be in pain - especially if his ears are down - and I am surprised the vet didn't prescribe any pain relief medication if they suspected a fall.
Try googling exotic vets for your area- there is usually one not too far away. They tend to specialise more in small furries and be more experienced with hamsters. Most good vets would think pain relief an important thing at his age. It's a drug called Metacam that is usually prescribed. If he hasn't improved I would suggest going to another vet for another opinion and at least some pain relief for him to help him keep comfortable. I know it costs, but we had the benefit of them throughout their lives, and in old age sometimes there are some costs involved.