Is there truth to this saying? That we should be idealist when young but realists when older?

No. It's just a convenient way to try to say you are better than others. I don't know of anyone who is conservative now who was a liberal when they were young.

It refers to the "original position". https://plato.stanford.edu/entries/original-position/

Older people who are successful tend to vote to solidify the position they have achieved in life, low taxes for the wealthy, few benefits for the poor, support policies that DECREASE social mobility so your kids and grand-kids stay on top, low inheritance, conservative when it comes to racial makeup of the country, etc...

Younger people who don't know how they'll end up in life tend to pick a more socialistic system, you don't know whether you will be successful or unsuccessful in life, sick or healthy, etc... so you try to cover your bets by supporting government programs that are more likely to avert personal risk or help you if you happen to be unsuccessful. Being young and not knowing how things will go for your life is considered to be the "original position".

The philosopher John Rawls, says that choosing government policies from the original position maximizes justice. In his short book "A theory of Justice" he outlines why in a very strong argument.

It's "catchy" but not much else.

I find that people become more institutionalized as they get older, which isn't the same as being realistic, and I say this as some on their 40's, people my age tend to embrace the system because its easier rather than try to change it.

No. It's the kind of thing conservatives tell themselves so they will feel superior to everyone else.

All who gain wisdom throughout a life of experience realize there is a lot of truth in that old saying.
But those results are not guaranteed.
If an individual spends their life being cared for by others they may never gain any real wisdom.
Wisdom is Not what makes an individual greedy. Those who function by pursuing only self-interest are not wise.
Many Socialists seek to Force others to do what the Socialists want so they don't have to take on those responsibilities themselves.

No. I dont believe this Is correct at all.

I think we should always be idealistic, but should strive to temper our idealism with realism. I think many people who are idealistic lose that quality as they become more world-weary with age, but that doesn't need to be the case. Those two qualities have nothing to do with political ideology.

What if I tell you I was a realist when I was young and an idealist now?

Would you think I grew up before my time or a man trying to recapture his youth?

Would you think this as being strange?

I'm one of those rare types who has always tried to be both.