18 years

My cousins - the oldest sibling was 17 when the youngest was born. The other siblings were 14 and 12 at the time, so a huge gap between #3 and #4. An acknowledged oops situation. Parents had married young, first baby in their early 20s, and I guess thought they were safe at nearly 40.

Another woman I know had a 14yo and an 11yo... and then had twins!! So not as big of a gap, but ... twins?!?!? (No fertility treatments, she was very open that this was NOT planned)

28 years, between my mom’s oldest aunt and youngest uncle from the same grandmother. My mom’s grandmother was pregnant with her youngest child at the same time as two of her daughters, so the uncle was actually younger than his nephews.

My mom is 25 years older than my aunt, who was born two months after I was. Aunt and I are more like cousins, and mom relates to her sister more like an aunt. My grandfather was 17 when my mom was born, 42 when my aunt was born: different moms.

22 years apart and to add to it, my cousin is older than my uncle.

A woman who works for me has siblings younger than her own thirtysomething aged children. She is one of 17 children; her mother was an extreme example of a Catholic baby-hoarder.

My mom (who was the oldest) was 17 when her youngest sibling was born. So that sibling (my aunt) is only 9 years older than me.

Her mom (my grandmother) was also the oldest, and was 18 when her youngest sibling was born. So that sibling (my mom's uncle) was only 6 years older than my mom.

My mom broke the cycle; I'm the oldest and only 3.5 years older than my youngest sibling.

27 years, but there were 12 kids in between them.

All full siblings, no multiple births. Catholic family with kids born from late 40s to late 70s.

My sister was 20 when she had her first child. My youngest sibling (well call him M) was 13 at the time. My parents raised her child (my nephew), and as a toddler, N, he would constantly assume M was his brother, not his uncle. He’d go around telling people “My broder M had a baseball game.” My parents would constantly correct N but it was fruitless until N was much older.

DH is 14 years younger than his older sibling. A high school friend is 20 years older than his youngest brother. Both are full siblings.

My cousin had her first at 16 and her youngest at 36. Different dads, though. She was pregnant with the youngest while her oldest had a baby on the way. Her youngest son and granddaughter are two months apart.

It’s not really a sibling at that age, it’s more like a cousin. Cousins can be close as adults if they choose, but they’re too far apart to have a typical sibling relationship.

My great-grandmother was the 2nd youngest in a big family. Her parents would have been early 40s when she was born and they even had another baby after that. Her oldest brother would have been 20-21 at the time.

25 years. TeEn mother

One of my aunts had 3 kids in the mid-80s, then had a fourth in 2001. I know another family with a similar spread. I’ve always been fascinated by this because it would be cool to have a sibling who grew up entrenched in a different generation than you (or at least I think it would). I have a couple first cousins who are about 25 years older than I, but I’ve never met them.