"How I Met Your Mother" ended its nine-season run last night . . . and some parts of the finale were filmed NINE YEARS ago.

LYNDSY FONSECA, who plays the "Mother's" daughter, says she and DAVID HENRIE filmed a bunch of stuff together back in 2005, so they wouldn't appear to age in the "flash-forwards" over the years.

She explains, quote, "We shot this last little section nine years ago.  It was so long ago, I don't really remember what I said.  David was growing so fast.  He grew like four shoe sizes and his voice dropped, so it was very apparent."

 

She says they filmed improvised scenes and generic reaction shots that could be used for any occasion throughout the course of the show.

 

David adds, quote, "When we started, I wasn't even old enough to drive.  Now, I come home and my cousins are watching the show.  It's always a funny feeling.  They always say, 'Baby David is on TV!'"

 

(SPOILER ALERT:  There's full recap on the events of the finale, here.)