The biggest issue I am getting is the age.
Who is giving you age issues? Parents? Friends? What? Are you having trouble coping with the age gap yourself, or the reaction to the age gap from other people? I am not clear.
But I am curious with how many othe poly-relationships have an age gap so wide
I know of one poly friend with a large age gap. 17 yrs? The rest I don't know the exact ages of but they seem closer in ages than that. In monoships? One of my aunts and uncles have a 17 year age gap. My parents have 7 yrs. They are all going strong.
I don't think age itself matters or even whether in a polyship or monoship kind of family. What matters is if the family members understand each other in context and put in the work to understand each other and put in the work into making sure the family is healthy.
You at 18 and wife at 21 puts you both in the age range where brain is still doing a lot of development. He is past that age.
I don't know how that will play out in term of being able to see each other's point of view, communicate, and deal with conflict resolution. Could be something to consider as the family travels Life's Journey together in this first decade. It's the same as considering your own aging on the other decade stages -- menopause for the women, andropause for the man. How does it affect family harmony as each family member hits ages/stages milestones?
There's also six maturities. More here in my blog thread.
Not everyone achieves them all at the same rate. Chronological and physical everyone does just by hitting the next birthday, but the rest one has to work at developing -- emotional, intellectual, social, philosophical. Where all three people in the family are at in the development of their maturities could also matter in family harmony.
Last but not least, the addition of children into a family changes family dynamics considerably. How long have you been seeing them and joined the family?
Given that your other thread
was talking about wanting to break it up, you could consider waiting a touch on the TTC thing.
Not just to learn how to get along better as a family unit first, or to better see how each one of the parents and would be helping to parent your own child, but to space out the family children out so it's easier to raise them. There's a reason for that common 3 year gap between siblings. Having a lot of babies under 3 yrs old in one house is taxing and while you'd have an extra parent in this family configuration, it's still challenging to family harmony. It's so much easier when the older ones have started VPK or elementary school so part of the day they are over there.
So again... I don't think age gap much matters. More is the willingness of the family members to understand each other in context and to pull their fair share of the responsibilities in service to the family. Then the family can function as a healthy, harmonious family unit -- pulling as a team.
My 2 cents,