eruvin 82

They were sitting opposite from me on the tube. It looked like a half-term shopping trip for teenage daughter and mum. She closed her eyes and fitted herself around her mother's body and held on. Every so often she took a sip from her mother's water bottle. Neither said a word to each other the whole journey. Just tenderly travelling together.

and yet the Sages asked "who is the child who does not need their mother? what age? 4 or 5, or 6....?"

but there seems to be no age limit. No tachum.

(this page discusses that a child is included in their mother's eruv and shabbat boundaries up until they no longer need their mother. The shabbat boundary called the tachum, and is the limit one is permitted to walk on shabbat)