Translation:When I saw him, he was still young.
No, I'm pretty sure that doesn't work. You use "als" to mean "when" for something that happened in the past and is now over. For example, "Als ich ein Kind war, spielte ich Fußball" - "When I was a kid, I played soccer." These links explain it more: http://goo.gl/8A5Kt6 and http://goo.gl/vUByxr (this one is kind of long and doesn't actually get to the point until more than halfway down the page, but I found it extremely helpful so I highly recommend reading it when you have the time). Hopefully this helps.
"war" is the main verb and it is in second position. "Als ich ihn sah" is a subordinate clause that is taking up first position.
- "Als ich ihn sah, war er noch jung."
You will see this with most opening words (Subordinating Conjunctions) like "weil" or "bevor" as well.
z.B.: Weil sie schmutzig sind, wasche ich mir die Hände
Because they are dirty, I am washing my hands
(Because they dirty are, wash me to myself the hands) lit.
Reminds me of Monty Python: "People called Romanes, they go, the house?"
And apart from "aber", "und" and a couple of others, these subordinating conjunctions push the verb to the end of the clause. Thus, "ich ihn sah" or "sie schmutzig sind" above.