There are people in this world who don't know who JTT is. Yeah ... let it sink in.

17-year-old Kendall Jenner had everyone feeling older than bricks with a simple tweet about Jonathan Taylor Thomas.

The "Keeping Up with the Kardashians" star was apparently going through old Tiger Beat with a friend and had no clue who the former "Home Improvement" hottie was.

"Only old ppl know who he is," she tweeted along with a photo of JTT in all his glory.

She has a point ... there's no reason a 17-year-old should know who he is, she was only 4 when "Home Improvement" went off the air in 1999.

But it still burns.

And we're about to make you feel even worse.

We decided to pull together some other facts and photo galleries that will make you feel ancient.

The first season of "The Real World" premiered over 20 years ago in 1992 ... and will have its 28th season this year. With a minimum casting age of 20, those who were born the same year the show debuted can audition.

"TGIF" first premiered over 25 years ago. Meaning all those cute kids on "Full House," "Step By Step," "Family Matters" and "Boy Meets World" are all much, much older (and in the Olsen Twins case, ridiculously richer).

"Boy Meets" World is getting its own spinoff this year, centering on Cory and Topanga's pre-teen CHILD.

"Jurassic Park" came out 20 years ago this year. The kid who played Timmy, he turns 30 in September ... and he's kinda hot now.

A 4th movie is currently in the works, wonder if he and Lex will return for more dino-drama!

And, probably most depressing of all, The Lost Boys of "Hook" became men. It's been over 21 years since the flick starring Robin Williams dropped in theaters and they all definitely grew up.

Check out all the Now & Then galleries above

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