I always see Ernie and Bert as "ageless" characters, not to the extent that they never age (which none of the Muppets do), but rather, they just have no calendar ages, period.
Some characters its easier to look at in that perspective, others not so much, but I feel Ernie and Bert fall into the category that it's easy to see them as not having any age at all.
As for the whole pesky little brother / uptight older brother type of relationship, there are some who feel that there initially they were trying to establish a bit of a father/son relationship, with Ernie being the son, and Bert as the surrogate father-figure.
Some characters its easier to look at in that perspective, others not so much, but I feel Ernie and Bert fall into the category that it's easy to see them as not having any age at all.
As for the whole pesky little brother / uptight older brother type of relationship, there are some who feel that there initially they were trying to establish a bit of a father/son relationship, with Ernie being the son, and Bert as the surrogate father-figure.