It all depends upon many factors and variables. It also depends upon the emotional age/intelligence of the oldest siblilng. It further depends upon if the oldest sibling is always expected to be a parentified child watching the younger sibling/siblings a la Duggar 24/7/365 or it is a sporadic/emergency occurrence. Lastly, another influential factor is the older sibling relationship to the younger sibling/siblings. Is the older sibling warm/loving/respectful towards the younger sibling/siblings or is he/she cold/hostile/angry/distant/abusive towards the younger sibling/siblings?
Some older siblings are so immature that they cannot be trusted to watch a sibling even at the ripe old age of eighteen. If a parent assigns that particular oldest sibling to watch a younger sibling/siblings, some type of catastrope could occur. Frankly, there are some older children, regardless of their age, who SHOULDN'T watch a younger sibling/siblings as it ISN'T in their nature to do so. Yes, there are teenaged oldest siblings who feel that watching younger siblings even for a short period is hampering their style.
Then there are older siblings who are abusive towards their younger sibling/siblings. Such siblings should not be trusted to be alone with a younger sibling/ siblings for any period of time. Have some other trusted relative/friend look after the younger sibling.
Then there is the older siblings who LOVES nothing better than to watch a younger sibling/siblings. Such siblings, regardless of age, are natural parents. They actually ENJOY being with & interacting with their younger sibling/siblings. If parents have such older children, TREASURE them for they are a rarity!
As regards to age, under 15 years of age is WAY TOO YOUNG to watch a younger sibling/siblings. Children under the age of 15 AREN'T often equipped emotionally, psychologically, nor mentally to watch a younger sibling/siblings for any period of time. However, there may be some exceptions depending upon an older sibling's innate maturity quotient and if he/she has an exceptionally stable psychological makeup.