Definition of a bare trust

Reading through TSEM1563, it’s not clear what is a bare trust and what defeats the immediate title to capital.
A testator named 4 grandsons to receive an 80% share “subject to each attaining 25 years of age in equal shares”
“If this fails, then the share is to pass to the survivors.”
As there are 4 named individuals, is this an indefeasable interest?
Or is it that they have to attain age 25, a condition before they become entitled?

I would read that as an interest contingent upon them attaining the age of 25, hence the gift over if one or more dies before that age. It is not a bare trust.

It may become a bare trust when each child either reaches 25 or dies under that age. Each share of a beneficiary who reaches 25 will be held on a bare trust but only if and to the extent that the trust assets do not comprise land, a controlling shareholding in a company or a mortgage debt (Lloyds Bank v Duker) and probably a valuable chattel or any other property that is indivisible either physically or only by depreciating the value of an individual share compared to a pro rata share of the value of the whole.

Jack Harper