There are many things one can do with this passage. You are right in saying "τέκνα is not a term that is age limited." Neither is the English word "children." And yet the image that almost always comes to mind when you hear the word "children" is little kids or adolescents. The word can absolutely refer to adults who are the children of their older parents, but such a concept would never come to mind naturally unless you apprehend the context of the word "children" to be that of adult children. But, perhaps Paul did mean adult children as well, so I'll suppose that in my interpretation. I also want to say something else before I move on to my interpretation of the given scripture.
In order to "obey" someone, they have to give you a command(an authoritative order). As children grow up, parents (ideally) tend to "command" their children less and less and instead give advice, counsel, and guidance. This is extremely optimal, as parents want their children to learn independence so that when they grow up they can start a family with a mate. You see, commands are replaced by guiding principles that a parent's child can(and should) take heed of to try to implement in their own lives. When a parent's child is young, the parent is head over that child; the children are subordinate, and as obliged to obey(so long as what they ask does not go against God, e.g. steal, lie/spread falsehoods, or even perform sexual acts) their parents(though, once again, less and less as they grow older; that is, if the parents try to help their children gain independence). But when a child is grown up(not 18. I'd say somewhere around 25, as a human's prefrontal cortex isn't fully developed until 25 years old), they are (if their parents raised them right) fully equipped(in the sense that it won't be a total catastrophe, not that they will do so perfectly) to start a family; a husband to become the head, and a wife to become the body. That is why it is written, a man shall leave his father and his mother, and shall cling to his wife, and they shall become one flesh. (Genesis 2:24) Ok, now I'm done.
The word for "obey" in Ephesians 6:1 is "ὑπακούω." Here's what Strong's Concordance says about this word; "from ὑπό(G5259) and ἀκούω(G191); to hear under (as a subordinate), i.e. to listen attentively; by implication, to heed or conform to a command or authority:—hearken, be obedient to, obey." So, it can mean "to obey", but it more generally means "to hearken, listen attentively." The same word, ὑπακούω, is used again in Colossians 3:20;
Children, obey your parents in everything, for this pleases the Lord.
At least in Ephesians 6:1, Paul added "in the Lord" to denote that he wants children to obey their parents only if what their parents say is in accord with the guiding principles of God(and, if that is the case, then we are actually obliged to obey our parents, no matter what age we are, as obeying them would be synonymous with obeying God Himself[and we are undoubtedly always obliged to obey God], e.g. if our parents tell us not to spread falsehoods about a coworker that we are mad at). Clearly, Paul must not mean that we are obliged to comply with everything our parents say. If they tell us to do something that goes against God, are we not obliged to disobey them? So, what is Paul getting at here(in both Ephesians 6:1 and Colossians 3:20)?
Well, all I think he is trying to get across here is that as children, having been raised by our (older) parents who (most likely) have much more wisdom and discernment than us(due to all they've experienced in life), we are obliged to listen attentively to what they say. We should never ignore them or brush them off when they want to give us some genuine advice, but we must always hearken to their advice, i.e. take it into great consideration. This, I think, is true for any and all ages.
Once again, this is presupposing that Paul had in mind "children of any and all ages" when he wrote either Ephesians 6:1 or Colossians 3:20, which I don't personally think he did. But perchance he did, and if so, the interpretation given above is the one I'd go with.
Hope this helps, and have a good day!