I think, during the sequences where we saw Clu (as the de-aged Bridges), it's understandable that there's some level of deresolution that takes away from the validity of the imagery....call it: in story computer rendering.
But I think the scenes where we see the Younger Flynn with his 7 year old son, Sam, the effect is quite convincing...especially when Flynn leaves and turns off the porch light. The light sourcing and reflection are really quite good.
Besides, think on it, in X3, the de-aging for Stewart and McKellen was on the order of 20 years...and given their actual ages at the time (easily from their mid-60's), that didn't seem to be too much of a demand.
But with Jeff Bridges and Bruce Boxleitner, taking them back 30 years from their current ages (late 50's early 60's) called for a bit more de-aging...and for the effort, I thought it was pretty considerable.