appropriate age question

I'm not quite sure what you mean by "too old".

In the stylistic sense, it's pretty obviously set in the late-80s or early-90s. All of the "hip" clothes are quite laughable now (but in fairness, someday the "sagging", baggy pants style will die away... alright, I'm looking forward to that, and I'm in my twenties, but I digress) The computers are all rocking Dos (and I think I saw the Amiga OS in some eps). There isn't a flat screen to be seen, CRTs all the way.

But the special effects, while mostly practical and camera-trick in nature, as opposed to CGI, generally hold up. The network/studio sank some serious cash into the show, and that in turn made for a really good production values.
http://www.youtube.com/watch?v=Fa9b7ZQ3Zkw

If the same production values had been put into a theatrical film around the same time, it wouldn't have been looked down upon on that front (ala Superman IV).

In terms of content, it had action and some relatively dark characters, but nothing that would make it "too old" for a seven year old. People die, invariably only by the bad guys, but it isn't gruesome or gory. By today's standards the action is pretty G/PG. There isn't any swearing beyond the occasional "hell" or "damn", and there is usual a strong, though not too heavy-handed moral to the episode (protect the environment, hate speech is wrong, homelessness affects children too, etc).

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This is a family friendly show. Yes there is plenty of action, but its never violent or vulgar.

I'd say the show is appropriate for a 7 year old, as long as he is interested in superheroes and fantasy/sci-fi.

You guys might enjoy certain episodes of Smallville that do feature versions of the Flash. The Kid Flash, Bart Allen/Impulse is really just a supporting recurring character, while Jay Garrick has a cameo presence in a certain movie event. Sadly the more well known ones like Wally West and Barry Allen don't pop up, though they are given a nod, the series was pretty reserved with exactly how to explore the Flash mythos. If you're interested in what eps to follow for the Flashes, check these:

Season 4 Disc 2: "Run" -- Introduces Bart Allen and Kal-El's first races with a Flash.

Season 5 Disc 4: "Vengeance Chronicles" -- Though only seen via archival materials, Bart has a presence in this transition to his story.

Season 6 Disc 4: "Justice" -- Establishes Bart as his Impulse alias concurrently with the Justice League's birth.

Season 6 Disc 2: "Justice & Doom" -- Bart's continuing adventures with the League are seen in their first mission around the world.

Season 8 Disc 6: "Doomsday" -- Bart and half of the League assist the Man of Steel with the concluding chapter to his first conflict with Doomsday.

Season 9 Disc 2: "Roulette" -- Though only present for an offscreen cameo, Bart does play a prominent role.

Season 9 Disc 3: "Absolute Justice" -- The history of the JSA is explored here as the heroic legacy of Jay Garrick is glimpsed.

Season 9 Disc 6: "Salvation" -- Albeit only barely glimpsed in an archival image during a JLA summit scene, Bart's presence is implied.

Season 10 Disc 3: "Icarus" -- While portrayed by a body-double, Bart does have a cameo at the very climax of this ep.

maybe I'm watching through different "filters" but aside from 1-2 jokes/romantic angles The show is pretty tame... Currently watching the DVDs and the 'worst' I've seen is in the 'Trickster' Origin Episode in which Barry is on the couch while a woman (hiding from the Villain) uses his Bedroom they have a romantic history and neither can sleep so they meet on the stairs, and Kiss...

--she makes a joke about how 'fast' he is and implies that they could have 'done it' in a fraction of a second
-- and then they move to the bedroom...

It's clearly implied that they proceed to do it off screen at normal speed
-- at 7 years old I doubt he'd get either implication (unless today's 7 year olds are more savvy than I was

you need to keep in mind that it aired on CBS in 1990... it predates the TV Ratings system, so the 'Big 3' were all pretty much playing it safe (and if I remember correctly at 9/8 central or Earlier in the so called 'family Hour'

I was around 10-ish when it first aired I certainly didn't find any thing 'mature' or Forbidden about it

If you believe that Tim Burton's "Batman" is not too far over your kid's head then this series would be great for him. It came out a year after Batman was in theatres and was shot in a similar vein albeit via TV. Some of the scenes, although more adult themed, were not too bad and could go over his head. The action would be considered tame by today's standards even though there is gun play, people getting shot...dying but not too overtly violent.
I'd let my 5 year old step-son watch this series, with me in the room with him to answer any questions he might have.

(looking at the time you asked this and realising I'm a few too late but hopefully your son didn't miss out on this show that should have been given a better chance)

I was nine years old when I saw this show and I loved it.

--
Hulk want hug kitties
But they so easy to squish
Hulk live in cruel world