First, the New York City market is a beast of a very different color from other markets around the country; thus, I want to confine my response to this question to Manhattan. Second, traditionally, the sweet spot for hiring in New York City is 3rd through 5th year, which just happens to be the time frame during which firms believe a candidate has garnered enough knowledge to be valuable and helpful to their clients but not so much knowledge/experience that there isn't additional room for growth. This range also happens to be a fairly comfortable range for firms in terms of lock step compensation - namely, not too high.
Having said that, the above statements do not mean that someone with experience above the five (5) year mark fails to be marketable. In contrast, a 6th year plus attorney, for example, is very marketable, but exactly how marketable will depend on the needs of a given firm. Where a firm is seeking a candidate who can hit the ground running and tend to clients directly, a candidate with at least six (6) years of experience will be immediately placement-worthy. More so, where a firm is seeking an ''expert'' in a given area, traditionally, they will ask for a 6+ year associate. Your question, however, relates to someone with even more experience, namely, at the 10+ year level. In addition, I would imagine that your question is colored by what is going on in the economy today, where a 10+ year lawyer is an awfully expensive individual.
To that end, I will say that a 10th year practitioner in New York City is marketable, but they will often be looked at by a firm more closely than other candidates because they, quite simply, cost more --- and firms are watching costs very closely right now. Thus, firms will often ask if such 10th year associate ''has portable business?'' If so, that lessens the risk for the firm of bringing this person on. If not, they will ask ''whether such candidate fits into the expert role within a given practice?'' If so, that helps. If not, or if the candidate needs to be trained in a new practice area, that makes such hire more difficult to swallow. Firms may also ask ''what such person requires going forward?'' This means --- is this candidate asking for fast track to partnership, a Counsel role, or other incentive-producing plan/title? If so, such candidate may require more of a commitment than a firm is willing to make right now.
These are all questions which start creeping into the mix as we gain more experience in our roles as attorneys and then venture out into the marketplace. Simply put, the more experience we gain, the more valuable we become, the more we cost, and, therefore, the more closely firms look at other determining factors when making hires at the senior end of the associate spectrum. So, in summary, you are still very marketable, but you need to hone in on the group where your particular skills will have real value and where your own career demands (title? compensation?) will match the firm's hiring goals for a given search. Although this may seem like a lot of extra hoops to jump through, the truth is that where there is a match, there is a hire.