Rob Lowe and Robert Wagner as Number Two in Austin Powers: The Spy Who Shagged Me
edited to add:
Damn, GreedySmurf. Great minds think alike - and apparently at teh same time!
IIRC, several of the women you saw were the actual players.
And as to “they look nothing alike,” check out Nightbreaker, a film where Emilio Estevez and Martin Sheen play younger and older versions of a character. They look a hell of a lot like each other… 
Kate Winslett/ Judy Dench and Hugh Bonneville/Jim Broadbent in Iris.
As is traditional in these kinds of threads I will push the rules set out by the OP and mention a TV show. In an episode of Magnum P.I. Jose Ferrer and June Lockhart play old flames separated since WWII. In flachbacks they are played by Miguel Ferrer and Anne Lockhart. It worked pretty well.
Mayim Bialik played a young Bette Midler in Beaches
Tarek Sharif played the young Yuri Zhivago.. His father Omar played Dr.Zhivago (not to be confused with Omar Epps playing Dr. Eric Foreman, who is not Eric Foreman of That 70’s Show, though the later Eric’s father guest starred on House).
Didn’t Tobey Maguire play a young Woody Allen in Deconstructing Harry? I think that he was a character in his book, but the character was basically him as a young man. Am I remembering that right? It’s been a long time.
There was also an (uncredited, acc. to IMDb.com) very small actor playing a newborn Spock - who was scorned by Sarek - in the execrable Star Trek V: The Final Frontier.
Terence Stamp played younger and older versions of himself - in footage filmed decades apart, for a different movie entirely! - in The Limey.
You guys and your high-brow movie viewing tastes.
Jamie Lynn Spears played the younger version of her sister Britney’s character in Crossroads. (Dajine Colon and Crystal Milton also played younger versions of Zoe Saldana and Taryn Manning in the same scene.)
I didn’t like that movie much, but that effect was cool!
Cary Elwes and Jon Voight played the same somewhat noteworthy Catholic dude, young and old, in the 2005 CBS TV movie Pope John Paul II. Voight was nominated for an Emmy.
Sorry, you’re not allowed to make up movies that never existed.
If you count tv series, then in HBO’s Rome, Octavian was played by Max Pirkis and then Simon Woods. Lucius was played by Marco Pollack, Alessio Cuna and Stefan Brown. Vorena the younger was played by Anna Fausta Primiano and Valery Usai.
Morgan Freeman’s son played a teenaged Red (only in a photo) with Freeman the Elder playing the, well, elder Red in The Shawshank Redemption.
On that same aside, does anyone know why all the Star Trek movies are even numbered?
Jena Malone played the young version of Jodie Foster’s character, Ellie Arroway, in Contact. Not sure if Malone counts as ‘fairly well known’ until later – although I’m pretty sure she’d already made Bastard Out of Carolina – but it doesn’t seem like others are sticking to that provision of the OP, so what the heck. 
Then there’s Wil Wheaton and Richard Dreyfus in Stand By Me. Also, Peggy Ann Garner/Joan Fontaine in Jane Eyre.
This years movie Evening features a couple of pairings; Claire Danes/Vanessa Redgrave and Mamie Gummer/Meryl Streep.
Well, I did say that it was execrable. Even if, as you suggest, it never existed. 
Another one: George Bailey and his buddies were shown, young and old(er), in It’s A Wonderful Life. Not sure if any of the child actors went on to greater Hollywood fame.
You could say that about a lot of the characters in a Woody Allen film.
But the son’s not very well known.
I’ve seen Final Frontier exactly once, just to be able to say I saw it, and I’ve blocked it out of my mind so thoroughly that I don’t remember that scene. Or most of the scenes. I actually walked off and started browsing the internet while the movie was playing, so really I haven’t even seen the end.
What film was the “different movie entirely”?*
*Yes, I know, I could look it up on Wikipedia, but I have to get my schoolwork done soon and Wikipedia is a trap.