Wouldn't the simpler solution be to simply reduce the pensions by the amount of UBI?

For example, a pensioner gettin $40k/year would get $25k under a $15k UBI. You would have to get them to agree to the renegotiation, so I suppose it's not a realistic solution.

Yes it would, although getting people to give up something is hard. Personally I would structure a UBI as a large negative income tax to try and avoid these problems.

Why should people with pensions not get UBI? They saved their money into a pension vs into a bank account.

Unless you are talking about replacing Social Security, but even there the math isn't as straightforward.

As for 18 and under, I think it's a bit tough. A 6 person household with 4 kids needs much more than a 2 person household. If we don't supply UBI to the children / wards of the children then we risk needing to recreate social assistance programs. Although it does run the risk of having kids for money, which admittedly probably already happens.

I was making the point that a UBI can have age exclusions and still be simple. I would be reluctant to provide a full UBI to children since they don't have the freedom to spend the money themselves.